Solid-state relaxations in linear low-density (1-octene comonomer), low-density,and high-density polyethylene blends

Extensive thermal and relaxational behavior in the blends of linear low-density polyethylene (LLDPE) (1-octene comonomer) with low-density polyethylene (LDPE) and high-density polyethylene (HDPE) have been investigated to elucidate miscibility and molecular relaxations in the crystalline and amorphous phases by using a differential scanning calorimeter (DSC) and a dynamic mechanical thermal analyzer (DMTA). In the LLDPE/LDPE blends, two distinct endotherms during melting and crystallization by DSC were observed supporting the belief that LLDPE and LDPE exclude one another during crystallization. However, the dynamic mechanical β and γ relaxations of the blends indicate that the two constituents are miscible in the amorphous phase, while LLDPE dominates α relaxation. In the LLDPE/HDPE system, there was a single composition-dependent peak during melting and crystallization, and the heat of fusion varied linearly with composition supporting the incorporation of HDPE into the LLDPE crystals. The dynamic mechanical α, β, and γ relaxations of the blends display an intermediate behavior that indicates miscibility in both the crystalline and amorphous phases. In the LDPE/HDPE blend, the melting or crystallization peaks of LDPE were strongly influenced by HDPE. The behavior of the α relaxation was dominated by HDPE, while those of β and γ relaxations were intermediate of the constituents, which were similar to those of the LLDPE/HDPE blends. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1633–1642, 1997     

Effect of conductive particles on the mechanical,electrical, and thermal properties of maleated polyethylene

Inclusion of conductive particles is a convenient way for the enhancement of electrical and thermal conductivities of polymers. However, improvement of the mechanical properties of such composites has remained a challenge. In this work, maleated polyethylene is proposed as a novel matrix for the production of conductive metal–thermoplastic composites with enhanced mechanical properties. The effects of two conductive particles (iron and aluminum) on the morphological, mechanical, electrical, and thermal properties of maleated polyethylene were investigated. Morphological observations revealed that the matrix had excellent adhesion with both metal particles. Increase in particle concentration was shown to improve the tensile strength and modulus of the matrix significantly with iron being slightly more effective. Through‐plane electrical conductivity of maleated polyethylene was also substantially improved after adding iron particles, while percolation was observed at particle contents of around 20–30% vol. In the case of aluminum, no percolation was observed for particle contents of up to 50% vol., which was linked to the orientation of the particles in the in‐plane direction due to the squeezing flow. Inclusion of particles led to substantial increase (over 700%) in the thermal conductivities of both composites. The addition of high concentrations of metal particles to matrix led to the creation of two groups of materials: (i) composites with high electrical and thermal conductivities and (ii) composites with low electrical and high thermal conductivities. Such characteristics of the composites are expected to provide a unique opportunity for applications where a thermally conductive/electrically insulating material is desired. Copyright © 2015 John Wiley & Sons, Ltd.     

Characterization of linear low-density polyethylene: Cross-fractionation according to copolymer composition and molecular weight

The structure of ethylene copolymers modified by α-olefins has become an area of intense investigation since the successful commercialization of so-called linear low-density polyethylene (LLDPE) resins. The molecular structure of a series of typical commercial LLDPE copolymers was investigated and compared to LDPE and HDPE. The commercial LLDPE resins studied contained about 7% by weight of butene-1. The resins were fractionated according to short-chain branching content by a technique called temperature rising elution fractionation. Size exclusion chromatography, x-ray diffraction, ¹³C nuclear magnetic resonance, intrinsic viscosity, and differential scanning calorimetry were used to fully characterize the whole polymers as well as fractions of a selected LLDPE resin. A broad set of data was assembled in this work to investigate the short-chain branching, long-chain branching, and the molecular-weight distribution of these commercial resins. The melting behavior of the LLDPE resins was found to be strikingly different from that of LDPE and HDPE. The broad and multimodal melting envelope of the LLDPE resins was found to be due to a broad and multimodal short-chain branching distribution. No significant long-chain branching was found in the LLDPE resins. The short-chain branching was found to decrease with the increase of molecular weight in a typical commercial LLDPE resin. The unique physical properties of these resins are certainly strongly controlled by the expression of the distinctive heterogeneous comonomer incorporation in the solid-state morphological structure. The physical and mechanical properties of these materials should be ultimately understandable on the basis of the unique morphology which results from the extremely heterogeneous incorporation of modifying α-olefin in these commercial LLDPE resins.     

Characterization and properties of polyethylene blends II. Linear low-density with conventional low-density polyethylene

Melting and crystallization phenomena in blends of a linear low-density polyethylene (LLDPE) (ethylene butene-1 copolymer) with a conventional low-density (branched) polyethylene (LDPE) are explored with emphasis on composition by differential scanning calorimetry (DSC) and light scattering (LS). Two endotherms are evident in the DSC studies of the blends, which suggests the formation of separate crystals. Light-scattering studies indicate that the blend system is predominantly volume filled by the LLDPE component whereby the LDPE component crystallizes as a secondary process within the domain of the LLDPE spherulites. In contrast to those of the LLDPE/HDPE blends, the mechanical and optical relaxation behavior of the LLDPE/LDPE blends are dominated by the LLDPE component in the vicinities of γ and β regions, whereas the trend reverses at high temperature α regions. This observation is accounted for on the basis of the relative restrictions imposed by the deformation of spherulites (which are primarily made up of the LLDPE component) at different time scales.     

Study of thermal and dielectric behavior of low-density polyethylene composites reinforced with zinc oxide whisker

Different scanning calorimetry and dynamic mechanical analysis are used to study the thermal behavior of composites by melt-mixing low-density polyethylene (LDPE) matrix and zinc oxide whisker (ZnO_w) fillers. Micrographs of the composites illustrate that needle or wedge shaped ZnO_w are distributed uniformly in the LDPE matrix. Dielectric properties of the composites are measured in a frequency range of 1-10 MHZ. The results show that the addition of ZnO_w does not affect the melting behavior of LDPE, but has an important effect on the heat of fusion, dynamic mechanical behavior, and dielectric behavior of the composites. This revised version was published online in July 2006 with corrections to the Cover Date.     

Nucleation Activation and Spherical Crystals Morphology of LLDPE/LDPE/TiO2 Nanocomposites Prepared by Non-Isothermal Crystallization

The nucleation activation of TiO₂ nanoparticles in the linear low-density polyethylene (LLDPE)/low-density polyethylene (LDPE)/TiO₂ nanocomposites prepared by non-isothermal crystallization, the spherical crystals morphology of the etched LLDPE/LDPE and LLDPE/LDPE/TiO₂ composites were investigated by differential scanning calorimetry (DSC) and field-emission scanning electron microscopy (FE-SEM), respectively. The results showed that the heterogeneous nucleation activation of TiO₂ nanoparticles was accelerated by the fast cooling rate. The spherical crystals in the LLDPE/LDPE and LLDPE/LDPE/TiO₂ composites were ascribed to the same crystal structure. It was worth to note that there was distinct difference between the morphology of the cocrystallization LLDPE/LDPE crystals and that of the independent crystallization LLDPE/LDPE crystals.     

Volume‐exclusion effects in polyethylene blends filled with carbon black,graphite, or carbon fiber

Conductive polymer composites possessing a low percolation‐threshold concentration as a result of double percolation of a conductive filler and its host phase in an immiscible polymer blend afford a desirable alternative to conventional composites. In this work, blends of high‐density polyethylene (HDPE) and ultrahigh molecular weight polyethylene (UHMWPE) were used to produce ternary composites containing either carbon black (CB), graphite (G), or carbon fiber (CF). Blend composition had a synergistic effect on electrical conductivity, with pronounced conductivity maxima observed at about 70–80 wt % UHMWPE in the CB and G composites. A much broader maximum occurred at about 25 wt % UHMWPE in composites prepared with CF. Optical and electron microscopies were used to ascertain the extent to which the polymers, and hence filler particles, are segregated. Differential scanning calorimetry of the composites confirmed that the constituent polymers are indistinguishable in terms of their thermal signatures and virtually unaffected by the presence of any of the fillers examined here. Dynamic mechanical analysis revealed that CF imparts the greatest stiffness and thermal stability to the composites. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1013–1023, 2002     

Roles of work of adhesion between carbon blacks and thermoplastic polymers on electrical properties of composites

The effect of the work of adhesion between carbon blacks and different thermoplastic polymers on the positive temperature coefficient (PTC) of composites was investigated. Thermoplastic polymers, such as EVA, LDPE, LLDPE, HDPE, and PP, were used with the addition of 30 wt% of carbon blacks. The work of adhesion based on the surface free energy of a composite was studied in the context of two-liquid contact angle measurements using deionized water and diiodomethane. It was observed that the resistivity on PTC behavior was greatly increased near the crystalline melting temperature, due to the thermal expansion of polymeric matrix. It was shown that the PTC intensity defined as the ratio of the maximum resistivity (rho(max)) to the resistivity at room temperature (rho(RT)) had the largest value on CB/HDPE composites. From the experimental results, the decrease in the work of adhesion induced by interactions between carbon black surfaces and polymer chains is an important factor in the fabrication of a PTC composite.     

Thermodynamic properties of liquid Au-Cu-Sn alloys determined from electromotive force measurements

A thermally conductive linear low-density polyethylene (LLDPE) composite with aluminum nitride (AlN) as filler was prepared in a heat press molding. Differential scanning calorimeter results indicated that the AlN filler decreases the degree of crystallinity of LLDPE, and has no obvious influence on the melting temperature of LLDPE. Experimental results demonstrated that the LLDPE composites display a high thermal conductivity of 1.25 W/m K and improved thermal stability at 70 wt% AlN content as compared to pure LLDPE. The dielectric constant and dissipation factor increased with AlN content, however, they still remained at relatively low levels, i.e., <5 in wider frequency range from 10 to 10⁶ Hz. The surface treatment of AlN particles had a beneficial effect on improving the thermal conductivity and dielectric constant, whereas, the dissipation factor was less affected. Additionally, the obtained AlN/LLDPE composites have possessed rather low dielectric constant and high electrical insulation, which is suitable for substrate and packaging materials.     

Chemiluminescence study on the oxidation of several polyolefins—I. Thermal-induced degradation of additive-free polyolefins

Chemiluminescence (CL) has been applied to evaluate the oxidation susceptibility of various polyolefins: low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE) and isotactic polypropylene (i-PP). The intensity of CL emission in inert atmosphere could be related to the previous oxidation level. The thermal stability at 170 °C of the hydroperoxides in LDPE seems to be lower than that in LLDPE or HDPE. The kinetic parameters of the oxidation at 170 °C in oxygen, calculated from CL data, suggest the following stability order: HDPE > LLDPE > LDPEi-PP. The intensity of CL emission was related to the CH₃ content as evaluated by Fourier transform infra-red spectroscopy.     

纤维状填料/HDPE复合体系的导电渗流与流变渗流行为的关系

研究了纤维状导电材料不锈钢纤维(SSF)填充高密度聚乙烯(HDPE)导电复合体系的导电渗流与流变渗流行为之间的关系,并与颗粒状导电颗粒炭黑(CB)/HDPE导电复合体系进行了比较.发现当SSF含量极低(0.3vol%)时,SSF/HDPE体系即发生导电渗流现象,且导电渗流转变区域极窄;而仅当SSF含量达到4.8vol%时,该复合体系才表现出流变渗流现象,这一结果与CB/HDPE体系及纳米级导电纤维填充体系截然不同.此外,通过正温度系数效应的研究发现SSF形成的导电通路稳定性高于CB/HDPE体系.我们认为,SSF/HDPE体系呈现的这些特点均与SSF较大的直径及长径比且其导电通路及流变渗流网络的形成机理不同有关.     

Effect of sol-gel derived nano-silica and organic peroxide on the thermal and mechanical properties of low-density polyethylene/wood flour composites

The influence of nano-silica, synthesized and mixed with low-density polyethylene (LDPE) through a sol-gel process, on the thermal and mechanical properties of LDPE and LDPE/wood flour (WF) composites, prepared in the absence and presence of dicumyl peroxide, was investigated. Scanning electron microscopic (SEM) analyses show a uniform dispersion of silica nano-particles of size 10-50 nm in the matrix, and Fourier-transform infrared (FTIR) spectroscopic results indicated interaction between the nano-silica and the LDPE matrix, which seems to improve for samples prepared in the presence of dicumyl peroxide (DCP). WF and nano-silica, as well as the presence of DCP during sample preparation, substantially improve the thermal stability of the LDPE matrix. The tensile strength of the samples decreased with increasing wood flour content, while the tensile modulus substantially increased. The presence of nano-silica gave rise to lower values for both tensile strength and tensile modulus, while higher tensile strength (and an increase in tensile strength with WF content) is observed for samples prepared in the presence of DCP. The tensile modulus increases with increasing WF content, but is not substantially influenced by the presence of nano-silica or by sample preparation in the presence of DCP. The DMA results were in line with the tensile results.     

Characterization of polyacetylene/low density polyethylene composites prepared by in-situ polymerization

The results of thermal, mechanical, and conductivity studies of polymer composites prepared by polymerization of acetylene in low-density polyethylene (LDPE) films impregnated with a Ziegler–Natta catalyst are presented and discussed. The enthalpy of melting of the LDPE component decreases as the amount of polyacetylene (CH)_x grown in the matrix increases and is rationalized as being due to entanglement effects, although grafting cannot be unequivocally ruled out. Composites prepared well below the melting point of the LDPE matrix possess enhanced tensile moduli and tensile strengths at break, compared with pristine LDPE; this suggests reinforcement of the LDPE amorphous regions by the (CH)_x chains. Measurements of electrical conductivity as a function of (CH)_x content for a series of iodine-doped composites reveal an apparent percolation threshold at ca. 3 wt % (CH)_x.     

Significant enhancement of electrical and thermal conductivities of polyethylene carbon nanotube composites by the addition of a low amount of silver nanoparticles

Electrically and thermally conductive high‐density polyethylene composites filled with hybrid fillers, multiwall carbon nanotubes (MWCNTs) and silver nanoparticles (Ag‐NPs), have been prepared in the melt state. The investigation of their electrical and thermal conductivities while comparing with high‐density polyethylene/MWCNT binary composites shows that the addition of only 3 vol% of Ag‐NPs does not reduce the electrical percolation threshold (P_c) that remains as low as 0.40 vol% of MWCNTs but leads to an increase in the maximum dc electrical conductivity of PE/MWCNT composites by two orders of magnitudes. Moreover, the association of both Ag‐NPs and carbon nanotube particles improved our composite's thermal conductivity. Copyright © 2014 John Wiley & Sons, Ltd.     

Adhesive effect of linear low-density polyethylene gels on polyethylene moldings

Adhesive effect of linear low density polyethylene (LLDPE) gels in organic solvents such as decalin, tetralin, and o-dichlorobenzene on high density polyethylene (HDPE) moldings has been investigated by shearing tests, and DSC measurements. For all of the gels the temperature at which the heated gel starts to exhibit the adhesive effect was about 70 °C, which is similar to the result of LDPE gel. In particular, when heated at 110 °C, LLDPE gel in tetralin showed such a strong bond strength that polyethylene plates of 3 mm in thickness and 20 mm in width gave rise to necking. It was found that LLDPE gel behaved as though it added LDPE gel to HDPE gel namely LDPE-like components in LLDPE resin exerted the adhesive effect at lower heating temperature, HDPE-like components exerted the strong adhesive effect at higher heating temperature.     

Phase behavior of blends of linear low density polyethylene and poly(ethene-propene-1-butene)

The aim of this work was the study of blends of linear low density polyethylene (LLDPE) and an ethene-propene-1-butene terpolymer (t-PP). Two types of polyethylene were used to prepare the blends: an ethene-co-1-hexene (LLDPE(H)) copolymer and an ethene-co-1-octene (LLDPE(O)) copolymer. These copolymers present similar comonomer contents, molar mass, molar mass distribution and catalyst systems, but differ in their comonomer distribution. The blends were obtained through mechanical mixing using a single screw extruder at different compositions: 20, 40, 50, 60 and 80 wt.% of LLDPE. From DSC measurements two separated melting and crystallization peaks were observed and dynamic mechanical analysis showed two glass transitions indicating that LLDPE/t-PP blends are immiscible in amorphous and crystalline phases in the solid state. X-ray diffraction showed that the unit cell parameters of both polymers in the blends remain unchanged independent of the composition of the blend.     

The effect of flame retardant-modified sepiolite nanofibers on thermal degradation and fire retardancy of low-density polyethylene

Flame retardant-modified sepiolite nanofiber (PSPHD-SEP) was fabricated through chemical grafting by introducing intumescent flame retardant oligomer (PSPHD) onto the surface of sepiolite fiber. Various sepiolite/low-density polyethylene (SEP/LDPE) composites have been prepared successfully via melt blending. The dispersion of various SEPs in LDPE matrix was observed by scanning electron microscope and transmission electron microscope. The thermal degradation behaviors of various SEP/LDPE composites with 3 mass% acid-modified sepiolite fiber (a-SEP) or PSPHD-SEP have been investigated employing thermogravimetric analysis/derivative thermogravimetry. The thermal degradation kinetics of neat LDPE, a-SEP/LDPE and PSPHD-SEP/LDPE systems was comparatively analyzed by means of Friedman and Flynn–Wall–Ozawa methods to further comprehend the effect of a-SEP and PSPHD-SEP on the thermal stability of LDPE. Due to the addition of PSPHD-SEP, the limiting oxygen index value of PSPHD-SEP/LDPE composite can reach 21.3%, and the UL-94V-2 rating is obtained. The cone calorimetry (CONE) tests showed that a reduced peak heat release rate can be achieved for PSPHD-SEP/LDPE composite accompanying with gas-phase fire retardant action.

     

Synergistic effects of zeolites on a novel intumescent flame-retardant low-density polyethylene (LDPE) system

A char forming agent and silica-gel microencapsulated APP were selected to form novel intumescent flame-retardant system (IFR) to prepare flame-retardant low-density polyethylene (LDPE) composites, and then the influence of zeolites on the thermal and flame-retardant properties of flame-retardant LDPE composites were studied. With the addition of 1 wt% zeolites to LDPE/IFR system, the LOI value increases from 29.0 to 34.0 %. The results of cone calorimetry show that the heat release rate peak and total heat release of the intumescent flame-retardant LDPE composite with 1 wt% zeolites decreases remarkably compared with that of without zeolites. The scanning electron microscopy indicates zeolites with suitable content can improve the quality of the char layer of flame-retardant LDPE composite which is more coherent and dense. The zeolites with the appropriate content can remarkably improve the flame-retardant properties of the LDPE composites.     

Investigating the mechanical,thermal and melt flow index properties of HNTs – LLDPE nano composites for the applications of rotational moulding

Linear low density polyethylene (LLDPE) is the one of the most popular polymer used for rotational moulding applications such as storage tanks. But, its inferior mechanical properties and thermal stability restrict the longer service. Hence, this study experimentally demonstrates the effect of Halloysite Nanotube (HNTs) concentration on LLDPE composites for enhancing the mechanical and thermal stability. HNTs were uniformly dispersed with LLDPE matrix through ultra-sonication, followed by compression moulding used to prepare the nano composites plates. The prepared composites are shown 19.2% improved tensile strength for 2 wt% HNTs, whereas 28.9% hike in flexural strength observed for 4 wt% HNTs composite, compare to neat LLDPE. Which shows that higher concentrations of HNTs is favourable in improving the flexural strength rather than tensile properties. In addition to that, higher concentrations of HNTs are also helping in improving the storage modulus of the LLDPE composites. The increase in mechanical properties mainly attributed due to effective load carriers (HNTs) in the composite. Besides, HNTs were also contributing for improving the melting point and residual char of the composites, which is indeed for storage tanks durability. The prepared composite was thermally stable at higher temperature up to 230 °C, because of HNTs chemical structure, the inner layer of HNTs constitute with Al₂O₃ and outermost layer constitute with SiO_2, both are thermally stable. Stated enhancement proves the potential effect of HNTs reinforcement in the LLDPE composite for rotational moulding applications.     

Thermo-mechanical degradation of polypropylene, low-density polyethylene and their 1:1 blend

Thermo-mechanical degradation of isotactic polypropylene (iPP) and low-density polyethylene (LDPE), and the effect of the specific degradation processes of each polymer on the degradation of a 1:1 blend of these polymers, have been studied by using a conrotatory double-screw mixer coupled to a torque rheometer. The products were characterized by Fourier transform infra-red spectroscopy and thermogravimetry under inert and oxidative atmospheres. Data for the blend were compared with curves simulated from a combination of the results for the single polymers. LDPE is shown to form three to four times more carbonyl-containing products than iPP in the processing conditions used to prepare the blend. The main reason for this difference are the stabilizers added to iPP by the producer before its pelletization. Blending with LDPE stabilizes iPP, even at temperatures well above its melting point.