Effect of zone drawing accompanied with crosslinking on the structure and properties of ultrahigh molecular weight polyethylene gel film

To enhance the thermal properties of ultrahigh molecular weight (UHMW) (viscosity-average molecular weight of 6 × 10⁶) polyethylene (PE) gel film, this was crosslinked by dicumyl peroxide (DCP) during a high-temperature zone drawing, which is effective to orient film. Through a series of experiments, it turned out that crosslinking actualized by an optimum amount of DCP and high-temperature zone drawing technique caused significant changes in the structure and properties of UHMW PE gel film. That is, crosslinking increased storage modulus of UHMW PE gel film at 25°C, resulting in improving thermal properties of the film. On the contrary, the crosslinking effect played a hindering role in raising the draw ratio of UHMW PE gel film. Maximum storage modulus of 165 GPa at 25°C could be obtained at the draw ratio of 324 of uncrosslinked homo-PE gel film. In the case of crosslinked PE gel film, the highest storage modulus at 25°C reached 65 GPa at maximum draw ratio of 150. Crosslinked film exhibited high modulus, even at 190°C, to some extent, while uncrosslinked homo-PE gel film was molten completely at 150°C. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1583–1590, 1997     

Thermal characteristics and temperature profile changes of structurally different polyethylenes with peroxide modifications

Crosslinking and processing characteristics of polyethylenes (PEs) with different molecular architectures, namely high‐density polyethylene (HDPE), linear low‐density polyethylene (LLDPE), and low‐density polyethylene (LDPE), were studied with regard to the effects of peroxide modifications and coolant flow rates. Dicumyl peroxide (DCP) and di‐tert‐butyl peroxide (DTBP) were used as free‐radical inducers for crosslinking the PEs. The characteristics of interest included normalized gel content, real‐time temperature profiles and their cooling rates, exothermic period, crystallinity level, crystallization temperature, and heat distortion temperature. The experiments showed that LDPE exhibited the highest normalized gel content. The real‐time cooling rates, taken from the temperature profiles for all PEs before the crystallization region, were greater than those after the crystallization region. The cooling rate of the PEs increased with the presence of DCP, whereas the crystallization temperature of the PEs was lowered. The HDPE appeared to show the longest exothermic period as compared with those of the LLDPE and LDPE. The exothermic period showed an increase with increasing coolant flow rate, but it was decreased by the use of DCP. As for the effect of peroxide type, the gel content and cooling rate of the PE crosslinked by DCP were higher than those for the PE crosslinked by DTBP. The DTBP was the more effective peroxide for introducing crosslinks and simultaneously maintaining the crystallization behavior of the PE. J. VINYL ADDIT. TECHNOL., 20:80‐90, 2014. © 2014 Society of Plastics Engineers     

化学交联PE-LD/SBS共混物的形状记忆效应研究

分别用3种不同苯乙烯含量的苯乙烯-丁二烯共聚物(SBS)与低密度聚乙烯(PE-LD)混合制成共混物,采用过氧化苯甲酰(BPO)为引发剂,使共混物发生化学交联形成网状结构,对交联共混物的交联度进行了分析,测试了交联共混物的拉伸强度与断裂伸长率,探讨了交联共混物的形状记忆效应。结果表明,在PE-LD/SBS共混物中,随着BPO含量的增加,共混物的交联度增大;BPO含量固定时,随着SBS中丁二烯含量的增大共混物的交联度增大;随着交联度的增加,共混物的拉伸强度与断裂伸长率均下降;当BPO含量为0.5 %～1.5 %时,交联共混物具有良好的形状记忆效应。     

Silane grafting and moisture crosslinking of polyethylene: The effect of molecular structure

The silane grafting and moisture crosslinking of different grades of polyethylene have been investigated. Three types of polyethylene (HDPE, LLDPE, and LDPE) with different molecular structures and similar melt flow indices were selected. The initiator was dicumyl peroxide (DCP), and the silane was vinyltrimethoxysilane. The grafting reaction was carried out in an internal mixer. The extent of grafting and the degree of crosslinking were determined, and hot‐set tests were carried out to evaluate the crosslink structure of the different polyethylenes. The LLDPE had the highest degree of grafting, while the LDPE had the least. The rate of crosslinking for LDPE was higher than that of HDPE and LLDPE. The gel content of LDPE was higher than that of HDPE and LLDPE. Hot‐set elongation and the number‐average molecular weight between crosslinks (M_c) were lower for LLDPE and LDPE than for HDPE. Increasing the silane/DCP percentage led to peroxide crosslinking, thereby decreasing the M_c and hot‐set elongation. The number‐average molecular weight (M_n), molecular weight distribution, and number of chain branches were the most important parameters affecting the silane grafting and moisture crosslinking. J. VINYL ADDIT. TECHNOL., 2009. © 2009 Society of Plastics Engineers     

Assessment of hindered phenol antioxidants on processing stability of peroxide‐cure LDPE by rheology and DSC analysis

Influence of commercial phenol antioxidants Irganox 300, 1010, 1035, and 1076 on peroxide‐cure reaction of low‐density polyethylene (LDPE) was evaluated through isothermal dynamic rheological and nonisothermal differential scanning calorimetry (DSC) testing. The results indicated that phenol antioxidants could reduce storage modulus of LDPE completely crosslinked at 175°C while they have a neglectable effect on gel fracture and activity energy of crosslinking reaction. On the other hand, time sweep dynamic rheological test revealed the antioxidants 1035 and 1076 with low molecular weight and low melting point could significantly depress scorch of crosslinkable LDPE at 135°C. The isothermal time sweep dynamic rheology test method was more sensitive than nonisothermal DSC test for characterizing the influence of phenol antioxidants on crosslinking kinetics of peroxide‐cure reaction of LDPE. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Molecular structure evolution in peroxide-initiated crosslinking of an ethylene vinyl acetate copolymer and a metallocene polyolefin elastomer

A combination of molecular theories and chemical reaction kinetics was employed to study the molecular structure evolution in the ethylene vinyl acetate and dicumyl peroxide, EVA/DCP, and metallocene polyolefin elastomer, m-POE/DCP crosslinking systems. Premixed EVA/DCP and m-POE/DCP compounds were cured at different temperatures and their gel contents were then measured. The values of crosslinking efficiency were evaluated and were then applied in the theoretical prediction of molecular evolution in a crosslinking process. The molecular weight distributions, gel time, relationships among gel content, degree of conversion, crosslinking density and processing time can all be predicted effectively. Information involving molecular structure evolution provides insights to practical process synthesis and material design.     

层压制备交联LDPE及其形状记忆性能的研究

采用层压成型的工艺,在低剪切应力下制备了交联低密度聚乙烯(XLPE)。对XLPE进行了凝胶含量、形状记忆性能与拉伸强度的测试。结果表明:当过氧化二异丙苯(DCP)质量分数为1.0%时,凝胶含量接近70%并趋于稳定,形状回复率达到100%,XLPE的拉伸强度提高。说明组合加工方法可显著提高DCP的交联效率,XLPE的形状回复与力学性能有明显的改进。     

Crosslinking of ethylene–octene copolymers under dynamic conditions: A new way to access polymeric hyperbranched structure

The crosslinking process of ethylene–octene copolymers by dicumyl peroxide (DCP) has been studied under static and dynamic conditions. Under static conditions, our results (gel time, insoluble fraction, and equilibrium modulus) qualitatively agree with the theories on the prediction of properties of random crosslink network. Furthermore, it can be concluded that the chain linking reaction is dominant and that the beta chain scission can be neglected for these copolymers. We report an original and new result: under dynamic conditions, inside the chamber of a batch mixer, the dynamically crosslinked samples are reprocessable, meaning that the final microstructure is totally different from that obtained under static conditions. Indeed, the samples crosslinked under shearing and mixing are soluble and the size exclusion chromatography analysis revealed a broad molecular weight distribution with a significant tail of high molecular weights situated around 10⁶ g/mol. This molten media can be imagined as a continuum of clusters of different multiscale sizes from the size of a free precursor chain to larger clusters of high molecular weights. Actually, the balance between reaction and mixing efficiency is the controlling parameter for the development of this hyperbranched structure under dynamic conditions. POLYM. ENG. SCI., 46:1530–1540, 2006. © 2006 Society of Plastics Engineers     

硅烷接枝热水交联聚乙烯共混物的研究

本文介绍了采用过氧化二异丙苯（ ＤＣＰ） 为引发剂,二月桂酸二丁基锡（ ＤＢＴＬ） 为催化剂,在双螺杆挤出机中硅烷（ ＶＴＥＳ） 与聚乙烯（ＰＥ） 共混物的接枝反应,以及通过热水交联获得硅烷交联聚乙烯共混物。研究了不同牌号聚乙烯的选取,正交实验优化配方以及热水交联时间对产品性能的影响。结果表明：选取适当的高密度聚乙烯,低密度聚乙烯及线性低密度聚乙烯共混物进行硅烷接枝交联,可获得具有良好的力学性能和良好的加工性能的交联ＰＥ 共混物;正交实验获得ＤＣＰ、ＶＴＥＳ 及ＤＢＴＬ 的最佳用量分别为０ ．１０ 份、２ ．５ 份、０ ．１５ 份;热水交联的合适时间约为１２ 小时,在此之前拉伸强度、维卡软化点及凝胶率随交联时间而升高,断裂伸长率随时间而降低,此后,几者均趋于稳定。     

过氧化物交联聚乙烯的研究

讨论了过氧化二异丙苯(DCP)交联聚乙烯(XLPE)的工艺、凝胶率对XLPE力学性能及结晶度等的影响。     

硅烷接枝交联LDPE、LLDPE及其共混物的结构研究

利用红外光谱、凝胶渗透色谱、热延伸试验、差示扫描量热法、扫描电子显微镜等方法研究了低密度聚乙烯（LDPE）、线型低密度聚乙烯（LLDPE）及其共混物的乙烯基硅烷接枝及交联产物的分子结构、熔融行为和形态。结果表明：硅烷接枝后，LDPE、LLDPE的重均摩尔质量小幅增加；硅烷接枝交联能力为：LLDPE〉LDPE／LLDPE共混物〉LDPE；接枝和交联使LDPE、LLDPE及其共混物的结晶度降低，晶粒变得不均匀；硅烷接枝和交联能增加LDPE／LLDPE共混物的相容性；交联结构提高了LDPE、LLDPE及其共混物的抗冲性。     

Preparation and properties of metallocene ethylene copolymer crosslinked by vinyltrimethoxysilane

Silane‐grafting and water‐crosslinking of ethylene–octene copolymer (EOR) was carried out. The influences of grafting formulation, comonomer content in EOR and time of the crosslinking process on grafting efficiency, degree and rate of crosslinking were investigated. The mechanical properties of various crosslinked samples are reported together with correlations with their gel contents. The results show that the extent of grafting increased with an increase in the amounts of dicumyl peroxide (DCP) and vinyltrimethoxysilane (VTMS) used in the reaction. The degree of crosslinking depended mainly on the extent of grafting, while the rate of the crosslinking process depended on the amount of amorphous phase in the samples. In this study, the variation of gel contents in the samples (0–77 %) had no significant influence on tensile properties. A large number of uncrosslinked regions are believed to exist inhomogeneously within the crosslinked polymers even after the sample has reached maximum gel content. The content of gel did not increase further when the crosslinking time was increased beyond that of reaching maximum gel, but the polymer network became denser, resulting in changes in polymer behaviour. Copyright © 2005 Society of Chemical Industry     

Phase dispersion–crosslinking synergism in binary blends of poly(vinyl chloride) with low‐density polyethylene: Entrapping phenomenon in PVC/LDPE/DCP blend

The co‐crosslinked products and the entrapping phenomenon that may exist in a poly(vinyl chloride)/low density polyethylene/dicumyl peroxide (PVC/LDPE/DCP) blend were investigated. The results of selective extraction show that unextracted PVC was due to not being co‐crosslinked with LDPE but being entrapped by the networks formed by the LDPE phase. SBR, as a solid‐phase dispersant, can promote the perfection of networks of the LDPE phase when it is added to the PVC/LDPE blends together with DCP, which leads to more PVC unextracted and improvement of the mechanical properties of PVC/LDPE blends. Meanwhile, the improvement of the tensile properties is dependent mainly on the properties of the LDPE networks. Finally, the mechanism of phase dispersion–crosslinking synergism is presented. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 1296–1303, 2003     

G.p.c. behaviour of hydrophobic beads with a double layer structure

Several types of hydrophobic gel beads, consisting of an outer crosslinked layer and an inner non-crosslinked or lightly crosslinked part, have been prepared. When the ratio of the thickness of the outer layer to that of the inner part of a gel was small and the difference between crosslinking densities of both layers was sufficiently large, polymers of high molecular weight were excluded from gel beads and polymers of low molecular weight penetrated into gel beads. Thus, eluting a mixture of homologous polymers through the column packed with the gels, using an organic solvent as an eluent, resulted in the polymers being fractionated into two groups. This unusual behaviour of the gels is very similar to that previously reported for hydrophilic poly(vinyl alcohol) gel beads (Hirayama et al.) with a double layer structure.     

Network chain density and relaxation of in situ synthesized polyacrylamide/hectorite clay nanocomposite hydrogels with ultrahigh tensibility

Nanocomposite hydrogels (NC gel) D-AM and S-AM were synthesized through in situ polymerization of acrylamide (AM) with hectorite clays of Laponite RD and RDS, respectively. The tensile performance of the NC gel was observed at different crosshead speeds and all of the NC gels exhibited an extremely high tensibility, e.g., the elongation at break even higher than 4000%, except for two samples with the lowest Laponite content of 1 w/v%. Strong tensile hysteresis was observed in the elongation-reversion curve, indicating a slow relaxation in the NC gels. Dynamic moduli G′ and G″ within linear viscoelasticity illustrated that the network structure was formed in these gels with the junction of Laponite platelets. The Laponite RD showed stronger gelation capability than the tetrasodium pyrophosphate modified Laponite RDS. The relaxation modulus G(t) for the NC gels was found to be similar to the slow rubber relaxation with the critical exponent n of about 0.16, much lower than 0.66-0.71 for the critical gel at the sol-gel transition. In comparison, the chemically cross-linked hydrogel showed almost no relaxation during the same time interval. The effective network chain density of the NC gel was determined from equilibrium shear modulus, which was evidently lower than that of the chemically cross-linked hydrogels. The present results reveal that the high deformability of these NC gels comes from their low effective network chain density with moderate relaxation.     

硅烷交联LDPE的微观分析

用扫描电子显微镜、凝胶渗透色谱、差示扫描量热法等分析了低密度聚乙烯(LDPE)接枝及交联前后微观结构的变化。结果表明,LDPE被接枝后交联,形成了明显的网状结构;LDPE2102TN00被硅烷A15l接枝后,交联形成的网状结构比采用2种硅烷A151／A171接枝具有更好的规整性。A151／A171接枝LDPE每11个分子中有1个硅原子。     

Percolation concept as applied to the degradation of elastomer networks

The kinetics of the degradation of elastomer networks are most conveniently disscussed by the stress relaxation which is due to the time change of the effective number of the network chain caused by thermo-oxidative scission and/or crosslinking reaction. However, this view is only correct in the initial stage of the oxidation. In the later stage, several number of elastically effective network chains simultaneously lose their effectiveness by the scission of only one network chain. In order to explain the network behavior in the course of degradation, we introduced a percolation concept to the degradation of the network chain. The important parameter of this concept is the critical exponent. We determined the critical exponent in the vicinity of gel to sol transition of ealstomer network by comparing the tensile modulus of DCP vulcanized rubber and the gel fraction. The experimentally determined critical exponent was in good agreement with the value which was calculated for the 3D percolation. From this point of view, we can construct a universal curve for the degradation. All the experimental stress relaxation curves fall in this universal curve by taking the critical point of degradation as a standard state.     

Thermal fractionation and crystallization enhancement of silane‐grafted water‐crosslinked low‐density polyethylene

Thermal fractionations performed using differential scanning calorimetry (DSC) to characterize the heterogeneities in molecular structures of low‐density polyethylene (LDPE), silane‐grafted LDPE (G‐LDPE), and silane‐grafted water‐crosslinked LDPE with gel fractions of 30 and 70 wt % are reported. In regular DSC analyses, LDPE, G‐LDPE, and the low gel fraction of crosslinked samples (30 wt %) give one broad endothermic peak at ～110 °C, whereas the high gel fraction of crosslinked samples (70 wt %) give overlapped multiple endothermic peaks in a much broader temperature range. After thermally fractionated in the range 60–145 °C, LDPE, G‐LDPE, and the low gel fraction samples give five to six endothermic peaks in the low‐temperature range, whereas the high gel fraction samples give nine peaks, with three additional peaks appearing in the high‐temperature range. These multiple peaks correspond to fractions of different molecular structures, with the additional peaks for the high gel fraction samples corresponding to the fraction of molecular segments with low or no branching. This fraction of molecular segments is increasingly extruded out of the gel region with increasing gel fraction by crosslinking and leads to an enhancement of crystallization of the sample. This crystallization enhancement behavior is also demonstrated by the X‐ray diffraction data and polarized optical micrographs. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 591–599, 2001     

Structure–property–processing relationships in chemically modified LDPE and LDPE/LLDPE blend

The properties of low-density polyethylene (LDPE) and its blend with linear low-density polyethylene LDPE : LLDPE (75 : 25) modified with varying concentrations of dicumyl peroxide (DCP) were compared. Chemical modification was carried out in a Brabender plasticorder under set conditions of temperature, rotor speed, and time. The results are reported in terms of the type of polyethylene used, concentration of DCP, mixing torque, temperature and time on the processing, and mechanical and physical properties of the modified samples. Interpretation of the results is given in terms of gel contents which relate to the three-dimensional network structure of polyethylene. The modified blend forms a denser network structure and, hence, better product properties relative to that of modified LDPE, indicating the influence of LLDPE in increasing the rate and extent of crosslinking on blending with LDPE. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:789–797, 1998     

Investigation of molecular structure of LDPE modified by itaconic acid grafting

Using thermomechanical spectroscopy, the molecular‐weight distribution and relaxation transitions have been investigated in commercial LDPE and grafted by itaconic acid (LDPE‐g‐IA). This grafting in the molten LDPE was done by reactive extrusion with varied content of reactants in the blend under alternating of a shearing rate applied onto the melt. The dependence of structural relaxation changes in LDPE is shown upon the depth of the mechanochemical transformations and the competing reactions at IA grafting, and also on the chemical crosslinking of the macromolecules. The reason for MWD bimodality for LDPE‐g‐IA obtained in dynamic mixing is the raised homogenization degree of the reactive blend and the higher grafted product yield compared with static mixers. The mixer type substantially affects the structure of the LDPE‐g‐IA amorphous phase. The data obtained reflect chemical transformations of LDPE molecules in IA's presence without an initiator of radical reactions (DCP). Most probable is the IA initiation of molecular crosslinking reactions. There could also occur IA thermodegradation and oligomerization. LDPE and IA or products of acid chemical transformations do not agree thermodynamically (the calculated solubility parameters are 16.1 (MJ/m³)^0.5 for polyethylene and 26.4 (MJ/m³)^0.5 for IA). From the above procedure it can be supposed that nongrafted IA (or its oligomers) exerts an antiplastifying effect on LDPE and LDPE‐g‐IA. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1771–1779, 1999