Crosslinking of poly(vinyl chloride) with bismaleic compound. II. Rheological behavior and mechanical properties of crosslinked PVC

In this study the rheological behavior of crosslinking PVC was studied. We carefully probed the effects of styrene and initiators on crosslink course and further verified PVC's crosslink mechanism suggested previously. St as auxiliary crosslinker was very important for PVC crosslink, no matter whether at lower temperature or at higher temperature, in the absence of initiator or in the presence of initiator. With higher decay temperature cumvl hydroperoxide (CHP) was a very appropriate initiator for PVC crosslinking. By controlling technological conditions, the crosslinked PVC with different cross density was obtained. We found that PVC with THF-insoluble fraction of about 7% had the best tenacity and maximum breaking energy. The tensile strength would increase as the gel-fraction content increased. By citing T. Kurauchi's ROF toughening theory, we satisfactorily explained the good ductility of crosslinked PVC with low cross density and the high fragility of over-crosslinked PVC. Because the motion of segments is hindered in thoroughly crosslinked PVC, its glass transition was so weak that we hardly found out T_g.     

聚氯乙烯化学交联研究进展

综述了采用辐射及化合物（二巯基－三嗪化合物、硅烷、二马来酸酐）化学交联聚氯乙烯的研究进展）。     

Polyisoprene,poly(styrene‐cobutadiene), and their blends. I. Vulcanization reactions with tetramethylthiuram disulfide/sulfur

Polyisoprene (IR), poly(styrene‐cobutadiene) (SBR) and IR–SBR blends were vulcanized with tetramethylthiuram disulfide/sulfur in a differential scanning calorimeter (DSC) at a programmed heating rate and isothermally in a press at 130^oC. The reaction was stopped at various stages, and the crosslink densities were measured. Residual curatives and extractable reaction intermediates were analyzed by high‐pressure liquid chromatography (HPLC). IR crosslinked more rapidly than SBR, and the difference was attributed to the greater reactivity of the accelerator polysulfides in intitiating reaction with IR than with SBR. In blends, the greater reactivity of IR led to the earlier crosslinking of IR, the depletion of curatives in the IR phase, and the diffusion of curatives from SBR to IR. Consequently, a zone of highly crosslinked material developed in IR close to the interface. The freezing point of a solvent, imbibed into a gel, is decreased as crosslinking proceeds, and dissimilarities in the crosslink densities of the phases in blends were demonstrated by comparing the crosslink density, calculated from swelling experiments, with the depression of the freezing point of the imbibed solvent. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1250–1263, 1999     

聚氯乙烯交联方法的研究进展

介绍了聚氯乙烯(PVC)的几种交联方法及其进展,包括辐射交联、化学交联、共混交联和PVC改性交联,重点讨论和对比了用过氧化物、硅烷和三嗪类化合物作交联剂的PVC化学交联,并指出共混交联法是最有前途的PVC交联方法.     

High‐speed photocrosslinking of thermoplastic styrene–butadiene elastomers

Photoinitiated thiol/ene polymerization was used to crosslink a triblock styrene/butadiene/styrene (SBS) polymer of low vinyl content (8%). The crosslinking process was followed by infrared spectroscopy (loss of unsaturation), insolubilization, swelling, and hardness measurements. The photogenerated thiyl radicals react with both the vinyl and the 2‐butene double bonds of the copolymer. Concentrations of less than 1 wt % in the trifunctional thiol crosslinker and in the acylphosphine oxide photoinitiator proved to be sufficient to create, within 0.5 s, a permanent chemical network in the elastomeric phase. This UV‐curing technology was successfully applied to crosslink rapidly commercial SBS–Kraton® thermoplastic elastomers. It proved also effective in the case of the much less reactive triblock styrene/isoprene/styrene (SIS) polymer which contains no vinyl double bonds. The thiol/ene polymerization was shown to be a much more efficient process to crosslink SBS and SIS thermoplastic elastomers than was the copolymerization of the rubber double bonds with a diacrylate monomer. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1902–1912, 2000     

热可逆交联剂对PVC力学性能及热性能的影响

用一种热可逆交联剂与聚氯乙烯(PVC)树脂熔融共混,研究了交联剂用量和交联时间对交联PVC制品力学性能和热性能的影响,分析了交联产物的热可逆转化行为。结果表明:热可逆交联剂能有效提高PVC制品的力学性能和热稳定性,当交联剂用量为3份时,PVC制品的力学性能达到最大值,维卡软化温度提高了4℃;PVC制品的凝胶含量随交联剂用量的增多而增大;交联时间为20 min时,PVC有较高的力学性能和热性能,更为重要的是PVC交联后具有热可逆转化特性。     

Dimethylammonium dimethyldithiocarbamate‐accelerated sulfur vulcanization. II. Vulcanization of rubbers and model compound 2,3‐dimethyl‐2‐butene

Rubber and model compound 2,3‐dimethyl‐2‐butene were vulcanized for various times with dimethylammonium dimethyldithiocarbamate [(dma)dmtc]‐accelerated sulfur formulations in the absence of ZnO. Model compound systems were analyzed by HPLC, and no reaction intermediates containing pendent groups were found. Crosslinked sulfides, characterized by ¹H‐NMR, were found to be essentially bis(alkenyl). Residual curatives were extracted from rubber compounds vulcanized for various times and analyzed by HPLC. Compounds, cured to various crosslink densities, were found to crystallize readily in a density column at subambient temperatures. This supports evidence from model compound systems that pendent groups are largely absent from vulcanizates. It is suggested that a reaction mechanism, similar to that applicable to zinc dimethyldithiocarbamate‐accelerated sulfur vulcanization, may be applicable with (dma)dmtc accelerated formulations. Very limited crosslinking occurred on heating compounds under vacuum, and this can be attributed largely to the rapid loss of (dma)dmtc from rubber at elevated temperatures. However, the slower rate of crystallization on cooling of the gels, compared to the rate in press‐cured vulcanizates of similar crosslink density, was interpreted as evidence that some pendent groups did form during heating with (dma)dmtc/sulfur. Crosslinking of such pendent groups may be inhibited by the loss of (dma)dmtc, that, like zinc dimethyldithiocarbamate, may catalyze their crosslinking, and/or to the loss under vacuum of dimethyldithiocarbamic acid that would form thiol pendent groups that would rapidly crosslink with thiuram pendent groups. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 3074–3083, 2001     

Rate constants of reactions of carboxymethylcellulose with hydrated electron, hydroxyl radical and the decay of CMC macroradicals. A pulse radiolysis study

The method of pulse radiolysis was used to study radiation-induced transformations of carboxymethylcellulose (CMC) of various degree of substitution (DS) in aqueous solution. The kinetics of the reactions of intermediate products of water radiolysis with this polymer indicated that hydroxyl radicals react very rapidly with the CMC, i.e. the rate constant is about 1×10⁹ dm³ mol⁻¹ s⁻¹ and independent of the DS of the CMC. On the other hand, the hydrated electron can be practically neglected as a precursor for macroradicals formation since its reaction rate constant with CMC is low, i.e. 4.0-5.0×10⁶ dm³ mol⁻¹ s⁻¹. As the result of OH radicals attack on the CMC, radicals on the main-chain, as well as on side groups (carboxymethyl) are formed. Environmental conditions determine the life-time of macroradicals, typically tens of minutes were detected. Most importantly, the pH level of the solution was observed to determine the preferential reaction pathway. Lowering the pH led to increased macroradicals recombination and gel formation.     

‘Living’ free radical graft copolymers,II: Structure

Poly(vinyl chloride)–n-propyl xanthate (PVC–nPX) macroinitiators with 3 to 14 bonded xanthate groups per molecule were synthesized using PVC and potassium n-propyl xanthate. The reaction took place rapidly between 30 and 45°C. Ultraviolet (UV) and nuclear magnetic resonance studies confirmed the presence of xanthyl groups on these macroinitiators. The PVC–nPX macroinitiators were grafted by methyl methacrylate under UV irradiation of 254, 302 and 336 nm producing graft copolymers and homopolymers. The molecular weights increased with increasing conversion, which is consistent with a ‘living’ polymerization process. The active species in these polymerizations are believed to be macroradicals and xanthyl radicals.     

Radiation crosslinking of poly(vinyl chloride)

The mechanism of the electron irradiation crosslinking of poly(vinyl chloride) (PVC) with polyfunctional acrylic monomers has been investigated as part of a program to develop an insulation for distributing-frame wire. These monomers were found to undergo rapid polymerization to form a rigid, three-dimensional network onto which PVC was grafted. Polyfunctionality was necessary for crosslinking to occur, since butyl methacrylate, containing only a single unsaturated bond, did not crosslink readily. On the other hand, trimethylolpropane triacrylate and trimethylolpropane trimethacrylate, each containing three unsaturated groups, gave extremely rapid crosslinking. Trimethylolpropane triacrylate showed 40 times the radiation sensitivity of tetraethylene glycol dimethacrylate, a diunsaturated compound. The rate of disappearance of unsaturation was related inversely to the rate of gel formation. This was attributed to immobilization of unreacted pendant double bonds in the rigid crosslinked network. In the triunsaturated monomers, half the vinyl groups were left unreacted when all of the PVC was insoluble.     

Radiation-convertible polymers from norbornenyl derivatives. Crosslinking with ionizing radiation

Vinyl polymers with pendant norbornenyl (bicyclo[2.2.1]heptenyl) groups crosslink rapidly on exposure to ionizing radiation. Analysis (according to Charlesby-Pinner theory) of extraction data from an ethylene–vinyl acetate copolymer and the corresponding ethylene–vinyl norbornenecarboxylate copolymer shows that the unsaturated polymer crosslinks by a chain reaction. At low doses, the kinetic chain length is about 4. Pendant norbornenyl groups accelerate crosslinking of a vinyl alcohol–vinyl acetate–vinyl chloride copolymer and convert poly(vinyl alcohols) and cellulose acetate from degrading to crosslinking polymers. A novel application of the Charlesby-Pinner plot strongly suggests linear dependence of the extent of crosslinking on the norbornenyl group concentration in a series of modified poly(vinyl alcohols). The greater effectiveness of pendant norbornenyl groups, compared to cyclohexenyl groups, demonstrates the importance of the reactivity of the double bond.     

Crosslinking of plasticized poly(vinyl chloride) by substitution and free radical reaction

Two new methods to obtain crosslinking plasticized poly(vinyl chloride) (PVC) are shown. One is by the substitution reaction of PVC with the sodium salt of γ-mercaptopropyl trimetoxysilane and the other is by the free-radical reaction of azide-modified PVC with γ-acryloxypropyltrimetoxysilane and vinyltri(2-metoxyetoxy)silane. The content of gel and the number average molecular weight between crosslinking (M_c) were determined by Soxhlet extraction and by using the Flory-Rehner equation. The reactions of PVC with the above organosilanes under normal processing conditions of the polymer lead to high gel contents and, therefore, low M_c. The ultimate tensile strength and elongation at break at 110°C of these polymers are greatly enhanced over those of the uncrosslinked polymer. The results are improved compared to those taken from literature for similar systems. © 1996 John Wiley & Sons, Inc.     

Influence of electron beam radiation on mechanical and thermal properties of styrene‐butadiene‐styrene block copolymer

Electron beam crosslinking of elastomers is a special type of crosslinking technique that has gained importance over conventional chemical crosslinking method, because the former process is fast, pollution free, and simple. The technique involves the impingement of high‐energy electrons generated from electron accelerators and the subsequent production of free radicals on target elastomers. These radicals result in crosslinking of elastomers via radical–radical coupling. In the process, some chain scission may also take place. In this work, a high‐vinyl (～ 50%) styrene‐butadiene‐styrene (S‐B‐S) block copolymer was used as the base polymer. An attempt was made to see the effect of electron beam radiation on the mechanical and thermal properties of the block copolymer. Radiation doses were varied from 25 to 300 kGy. Mechanical properties like tensile strength, elongation at break, modulus at different elongations, hardness, tear strength, crosslink density, and crosslink to chain scission of the irradiated samples were studied and compared with those of unirradiated ones. In this S‐B‐S block copolymer, a relatively low‐radiation dose was found effective in improving the level of mechanical properties. Differential scanning calorimeter and dynamic mechanical analyzer were used to study the thermal characteristics of the irradiated polymer. Influence of a stabilizer at different concentrations on the properties of S‐B‐S at varied radiation doses were also focused on. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Crosslinking of unfilled carboxylated nitrile rubber with different systems: Influence on properties

The presence of two functional groups, nitrile and carboxyl, in carboxylated nitrile rubber allows it to be crosslinked with different agents. In this study, we examined the influence of different crosslinking agents on the properties of unfilled carboxylated nitrile rubber. Significant differences were found when different crosslinking agents were used, as shown in the vulcanization curves, especially in the variation of the viscous component with the reaction time. The reaction rate was highest when organic peroxide was used, and it was lowest when metallic peroxide or copper sulfate was used. When the crosslinking agents led to the formation of ionic bonds (metallic oxide and metallic peroxide), the carboxylic groups ? COOH had a greater participation in the crosslinking. However, when copper sulfate was used, coordination bonds were formed, and the main contribution was due to the nitrile groups. Tensile strength, tear strength, and abrasion properties were superior when the crosslinking systems used led to the formation of ionic bonds. On the contrary, the compression set was optimum when covalent bonds were formed. Copper sulfate behaved as an intermediate between the two previous situations. The mechanodynamic response of the compounds also depended on the crosslinking agent used. The correlation between crosslink density by swelling in dichloromethane, maximum damping temperature by dynamic measurements, and glass‐transition temperature by differential scanning calorimetry had to be explained in terms of the crosslink type. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Enhanced reactivity of melamine–formaldehyde resins by fractionation. Crosslinking at ambient temperature

A commercial hexakis(methoxymethyl)melamine (HMMM) resin was fractionated (1) by crystallization of the hexane–soluble fraction from hexane and (2) by partitioning the hexane–soluble and hexane–Insoluble fractions between alumina and various solvents. Crystallization afforded hexakis(methoxymethyl)melamine (1) of 90 to 95% purity. Partitioning afforded 12 fractions with altered proportions of polar and nonpolar species; the least polar fraction was spectroscopically and chromatographically similar to the crystallized material. Relative reactivities of certain fractions were estimated by ¹H NMR studies of the rates of reaction with neopentyl alcohol in the presence of catalytic amounts of p-toluenesulfonic acid (p-TSA). It was found that the reactivity of fractions, rich in less-polar species, was substantially greater than that of the commercial HMMM resin, whereas reactivity of fractions, rich in polar species, was less. Reactivity was especially enhanced by removal of all active hydrogen species, notably those containing NH groups. A practical consequence of this result was that fully formylated and alkylated melamine resins could crosslink with polyols at substantially lower temperatures than the commercial HMMM resin, which contained NH groups. Therefore, room-temperature crosslinking was feasible.     

硅烷交联聚氯乙烯的研究

研究了硅烷的种类,增塑剂,交联时间等因素对PVC交联的影响,结果表明,通过调节硅烷的用量可以制备不同交联度的PVC,交联后的PVC其力学性能,尺寸稳定性,体积电阻率,热主为形温度得到提高。     

Greffages mecanochimiques sur le polychlorure de vinyle. II. Éatude des copolymègres grefféas

The copolymers obtained by mastication of poly(vinyl chloride) (PVC) in presence of methyl methacrylate and styrene monomers, and their fractions, were studied by using infrared spectroscopy, NMR, and flash pyrolysis. The degree of alternation between vinyl chloride and methyl methacrylate or styrene units is very low, and the copolymers are heterogeneous in composition. The results suggest that the mastication medium involves three kinds of domains: the inner part of the elementary grains of PVC is very slightly penetrated by the monomers. The polymerization initiated by the polymeric free radicals created by the rupture of bonds, begins in the surface layers of the grains and cause the formation of a true copolymer which accounts for 5–10% of the material. Light radicals arising from transfer reaction migrate into the third external domain which consists practically entirely of pure monomers and initiate polymerization.     

Crosslinking Reactions Occurring in Keratin Under Alkaline Conditions

Wool was treated with alkalis at various temperatures, and fractionation into α-, β- and γ-keratoses was used to detect crosslinking. At low temperatures crosslinking occurs to a small extent in the fractions of the wool usually isolated as γ-keratose. At higher temperatures more reaction occurs in that fraction of wool usually isolated as α-keratose. This latter reaction is concurrent with breakdown of the crystalline structure of the fibre. Analyses of the wool and its fractions indicate that lysinoalanine is formed as a crosslink.     

Melt free-radical grafting of glycidyl methacrylate onto polypropylene

This paper concerns free-radical grafting of glycidyl methacrylate (GMA) onto polypropylene (PP) in the molten state carried out in a batch mixer. This free-radical grafting proceeds very rapidly as it goes to completion within two to three minutes at 200°C. The grafting yield of GMA alone is low and it is greatly enhanced upon addition of styrene as a second monomer with a reduced chain degradation of PP. Homopolymer and copolymer of GMA formed during grafting were quantified. It is suggested that in the presence of styrene, the dominating grafting mechanism is that styrene reacts with the PP macroradicals to form more stable styryl macroradicals which then react with GMA to form GMA-grafted PP. The rate of this reaction is much greater than that between the PP macroradicals and GMA. Unlike grafting of mixtures of maleic anhydride and styrene, which is dictated primarily by formation of a charge transfer complex, free-radical grafting of GMA in the presence of styrene follows a random copolymerization process.     

亚乙基硫脲对PVC和PVC/ENR共混物的交联作用

从交联速率、压缩永久变形、凝胶质量分数、强伸性能和耐热性能等方面考察了亚乙基硫脲（ＥＴＵ）和ＥＴＵ／硫黄对ＰＶＣ及其与环氧化天然橡胶（ＥＮＲ）共混物的交联作用。结果表明,ＥＴＵ和ＥＴＵ／硫黄对ＰＶＣ及其与ＥＮＲ的共混物有明显的交联作用;交联后共混物的物理性能和耐热性能均有较大提高;适当增大ＥＮＲ用量,有利于提高共混物的热变形性能;ＰＶＣ／ＥＮＲ的共混比为７０／３０时,硫黄的最佳用量为１５份,促进剂选择促进剂ＤＭ／ＴＭＴＤ体系为佳。