Comparison of two curing agents in thermal crosslinking. I. Hard paraffin wax

The dynamic crosslinking of hard paraffin wax in the presence of dicumyl peroxide and potassium persulfate are investigated with differential scanning calorimetry, thermogravimetric analysis, infrared spectroscopy, and solubility analysis. The results indicate the occurrence of crosslinking, but there is an obvious difference in the respective crosslinking mechanisms in the presence of the two crosslinking agents. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 1551–1559, 1998     

Transitions and relaxations of linear polyesters related to poly(ethylene terephthalate). I. Melting

Observations obtained by other techniques were confirmed by DSC analyses. As the length of the methylene chain increased, T_m decreased, and crystallinity, heat capacity, and heat of fusion increased. The T_m and crystallinity of the isophthalate esters were lower than for the corresponding terephthalate esters. The values for an odd-numbered methylene sequence were lower than for the adjacent even-numbered members. A model was used to calculate the heat of fusion of theoretically 100% crystalline poly(hexamethylene isophthalate) and poly(decamethylene isophthalate).     

Flame retardant polyesters. I. Phosphorous flame retardants

Commercially available two types of phosphorous flame retardants (FRs) for polyester were compared from the viewpoint of chemical reactivity, reaction mechanism, and byproduct formation. 3‐(Hydroxyphenyl phosphinyl) propanoic acid (HPP) and 9,10‐dihydro‐9‐oxa‐10‐2,3‐dicarbonylpropyl)‐10‐phosphophenanthrene‐10‐oxide (DI) were used as a main chain type and a pendant type, respectively. HPP and DI showed different behavior with ethylene glycol (EG). More acidic HPP reacted with EG spontaneously and then produced esterified terminal hydroxy end group reacted with EG to form terminal diethylene glycol (DEG) end group. But the composition in the HPP solution was not varied for a long time below the esterification temperature. Contrast to HPP, DI showed normal esterification procedure. DI esterified with EG to form DI ester of EG, and DEG formation is not distinct. The difference of reactivity and DEG formation is caused by the difference of acidity in EG solution. For production of phosphorous containing polyester, it is desirable that HPP is introduced into the reactor in the mere solution state and DI in the esterified state. In addition, for production of polyester having the same phosphorous content in commercial scale, it is more beneficial to apply HPP than DI because of low input of FR due to low molecular weight. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Free volume-based modelling of free radical crosslinking polymerisation of unsaturated polyesters

A new mechanistic kinetic model is presented for the cure behaviour of unsaturated polyester (UP) resins. The model is based on free radical polymerisation mechanism and the free volume concept. The quasi steady-state assumption for the free radical concentration is not used, and the decrease in initiator efficiency with conversion and radical trapping are modelled separately. The glass transition temperature of partially cured samples was measured employing differential scanning calorimetry (DSC) in conjunction with dynamic mechanical analysis (DMA), and the values obtained were incorporated into the model. DSC obtained conversion-time data for a standard commercially available UP resin under isothermal conditions. The kinetic parameters of the model were estimated using parameter optimisation procedures resulting in good agreement between model predictions and experimental data. Modelling in combination with experimental cure data showed that at higher isothermal cure temperatures a greater extent of physical trapping of radicals occurs rendering them inactive.     

有机过氧化物交联剂

文中介绍了过氧化物交联、分解的机理及在橡胶中反应的全过程。     

Electron beam crosslinking of rigid-rod polyesters with flexible aliphatic side chains

The crosslinking of a series of thermotropic LC polyesters, consisting of a poly(p-phenylene terephthalate) backbone and flexible aliphatic side chains, by electron beam irradiation was studied as a function of the side-chain composition, side-chain length, and irradiation temperature. For a comparison of the effect of irradiation the G(S)/G(X) ratio, as determined from sol-gel or GPC measurements, was used. Crosslinking dominates if this value is smaller than 1, which is the case for four out of five polyesters studied. Only the polyester with hexyloxy side chains shows a slight tendency to degradation. Crosslinking is stimulated by longer side chains and the introduction of hetero atoms that are more easily ionizable, like oxygen. The introduction of an unsaturation has the largest effect and boosts the crosslinking process. Raising the irradiation temperature increases the tendency to crosslinking or degradation already present. For most of the polyesters studied, irradiation slightly improves the mechanical properties of cast films. © 1996 John Wiley & Sons, Inc.     

Chain extenders for polyesters. I. Addition-type chain extenders reactive with carboxyl end groups of polyesters

Effective chain extenders for linear polyesters were investigated among some bis-heterocycles, which were capable of coupling carboxyl terminals of the polyesters through addition reaction. Consequently, 2,2′-bis(2-oxazoline), 2,2′-bis(5,6-dihydro-4H-1,3-oxazine) and N,N′-hexamethylenebis(carbamoyl-2- oxazoline) were found to be the most effective chain extenders. Starting from a poly(ethylene terephthalate) (PET) having intrinsic viscosity ([η]) of 0.66 and carboxyl content (CV) of 46 eq/10⁶ g, one could obtain polyesters with [η] of above 1.0 and CV of below 5 eq/10⁶ g in the presence of the chain extenders. Typical reaction condition for the coupling of PET was heating PET under atmospheric nitrogen above its melting temperature with 0.5 mol % of a chain extenders only for several minutes. Bis-2-thiazolines showed no effect under the condition investigated, while in case of bis-2-imidazolines definite degradation was observed. Bis-N-acylaziridines and bisiminocarbonates resulted in some gell formation, indicative of side reactions.     

Bromostyrene-crosslinked polyesters. I. Thermal stability and flame retardancy

Bromostyrene was used as partial replacement of styrene for crosslinking polyester resins. The oxidative stability of the polymer was studied and the performance in standard flammability tests was tested. It was shown that flame retardation increased linearly with the Br content. Under certain conditions, self-extinguishing polyesters were obtained.     

Photopolymerization of dendritic methacrylated polyesters. I. Synthesis and properties

The so-termed starburst dendritic polyesters differ from classical polymers by their structural symmetry, high degree of branching, and high density of terminal functionality. The divergent synthesis of a series of polydisperse dendritic polyesters based upon pentaerythritol and 1,2,4-benzenetricarboxylic anhydride, modified by glycidvl methacrylate and methacrylic anhydride, is described, proceeding in a stepwise growth manner starting from an “initiator core,” building branched arms. These materials have about eight (two samples), 12, or 16 methacrylate double bonds (indicated as D-1-OH, D-1, D-2, and D-3, respectively) at the chain extremities on each molecule. The rheological behavior was investigated by measuring the dynamic viscosity of the dendritic polyesters. The viscosities of the dendritic polyesters largely decrease in the order D-1-OH, D-1, D-2, and D-3 at lower frequency, but the viscosities of D-1-OH and D-1 decrease dramatically at higher frequency. Using a viscometric cup, the flow time of the dendritic methacrylated polyesters was compared with epoxy acrylate-modified unsaturated polyester of a linear type and it was shown that the flow time for the former is only one-third of that for the latter. The thermal mechanical properties and glass transition temperatures of the UV-cured films increase with the degree of terminal functionality. © 1996 John Wiley & Sons, Inc.     

Free radical crosslinking of unsaturated bacterial polyesters obtained from soybean oily acids

Summary Poly(-3-hydroxy alkanoate) containing unsaturated side chains, PHA-soybean, were produced by feeding Pseudomonas oleovorans with soybean oily acids obtained from soybean oil. Unsaturation of PHA-soybean were found to be 10 mol-% of unsaturated side chains. Main saturated part of the biopolymer was Poly(3-hydroxy octanoate) with minor hexanoate and decanoate units. PHA films were crosslinked via free radical mechanism by means of thermally or under UV irradiation in the presence of benzoyl peroxide, benzophenon, and /or ethylene glycol dimethacrylate (EGDM). Crosslinking yield of the PHA films were found to be from 81 to 93 wt.-% from the sol-gel analysis. Swelling properties of the crosslinked PHA films in chloroform and toluene were also studied. Mc values of crosslinked PHAs were also calculated using Flory-Rehner equation. The crosslinked biopolyester obtained by thermally at 60 °C with benzoyl peroxide indicated the highest crosslinking density. Glass transition temperatures (Tg) of crosslinked biopolyester samples were changed from −33 to −45 °C while that of PHA-soybean was −60 °C. Received: 16 June 2000/Revised version: 22 January 2001/Accepted: 20 May 2001     

New crosslinking agents for vinyl polymers

The synthesis of diallyl esters of linear dicarboxylic acids, and their use as multifunctional crosslinking agents for vinyl polymers, was studied. The number of carbon atoms in the chains of the diacids used varied from 2 to 10 in order to define the effect of the chemical structure on the reactivity of the crosslinkers prepared. Measurements of swelling in chloroform were performed to determine the average molecular weight between crosslinks, thus making an overall evaluation of the crosslinking efficiency. Sorption experiments were also carried out, using aqueous ethanol.     

Correlation of viscoelastic properties with low-resolution 1H-NMR measurements of crosslinking unsaturated polyesters

The curing reaction of four synthezized and five commercial unsaturated polyester resins were characterized by low-resolution pulse ¹H-NMR and rheological measurements. The rheological measurements were done both by steady shearing flow and oscillatory shearing flow measurements. For the synthesized resins, which did not contain any inhibiting, these methods were quite comparable when measuring the gel time. Differences in measured gel times could be seen in the commercial resins. We concluded that the time taken from low-resolution pulse ¹H-NMR measurements was not actually the gel time, but the time at which the resins go from the microgel formation stage to the transition stage. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 671–680, 1998     

Reversible crosslinking in epoxy resins. I. Feasibility studies

The objective of the research is to investigate epoxy resins where crosslinks formed in curing reactions can be cleaved and reformed without significant sacrifice in resin properties. In this phase of the investigation, experimental disulfide-containing crosslinking agents have been studied, including dihydrazides of dithiodicarboxylic acids(aliphatic and aromatic) and dithio-aromatic diamines. Conditions for curing reactions and for subsequent cleavage of crosslinks by reduction have been determined in model systems and confirmed for a prototype epoxy resin. A preliminary comparative evaluation of cure kinetics and of crosslink density and thermal and mechanical properties has been carried out for this resin cured with aromatic curing agents (4,4′-dithiodianiline and methylene dianiline) and with aliphatic compounds (3,3′-dithiopropionic acid dihydrazide and hexamethylene diamine). It has been shown that in a resin fully cured with disulfide-containing crosslinking agents, crosslinks could be ruptured by reduction, and reestablished by oxidation under mild conditions.     

Radiation‐induced crosslinking of acetylene‐impregnated polyesters. II. Effects of preirradiation crystallinity,molecular structure,and postirradiation crosslinking on mechanical properties

Electron beam‐irradiated crosslinking has been studied in a series of acetylene‐impregnated polyesters and amorphous copolyesters, including poly(ethylene terephthalate) (PET), poly(butylene terephthalate) (PBT), poly(cyclohexane dimethylene terephthalate) (PCDT), and poly(cyclohexane dimethylene terephthalate‐co‐ethylene terephthalate) (P(CDT‐co‐ET)) having 29 and 60 wt % ethylene terephthalate (ET). The extent of crosslinking was observed by gel fraction measurements and was found to be significantly influenced by the aliphatic chain content of the polyesters (PET < PBT < PCDT). In addition, as the preirradiation crystallinity of the polyesters was reduced, the extent of acetylene‐enhanced crosslinking was greatly raised. Decreases in the postirradiation crystalline melting temperature and degree of crystallinity were observed in all the polyesters, using differential scanning calorimetry measurements. Particularly significant findings have been the shift in the glass‐transition temperatures (Tg) to higher temperatures and the decrease in loss tangents at higher temperatures, both of which confirm that crosslinking has taken place. The storage moduli (E′) in the rubbery plateau region of PCDT and P(CDT‐co‐ET) were significantly affected by irradiation dose. Increased network tightness in postirradiated PBT and PCDT films was also inferred from melt‐rheology measurements, in which stress relaxed more slowly following a stepped strain. Improvements in the mechanical properties of the irradiated polyesters and copolyesters were clearly evidenced by the increased modulus at higher temperatures, observed using dynamic mechanical thermal analysis and melt‐rheology methods. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4476–4490, 2006     

Benzocyclobutenes as crosslinking agents in synthetic polyolefins

Summary 4-Allylbenzocyclobutene was synthesized and copolymerized with 1-hexene and a non-conjugated diene, and the resulting terpolymer was cured thermally, under pressure, in the presence of carbon black. Mechanical properties of the crosslinked polymer containing allylbenzocyclobutene with two different dienes (5-methyl-1,4-hexadiene and 7-methyl-1,6-octadiene) were studied and are presented. Similar properties of a standard sulfurcured polymer composed of 1-hexene and the two dienes were measured for comparison.     

Emulsification properties of polyesters and sucrose ester blends I: Carbohydrate fatty acid polyesters

A number of carbohydrate fatty acid polyesters, potential fat substitutes, were screened for their ability to reduce surface and interfacial tensions alone or as blends with commercial emulsifiers. Commercial sucrose ester emulsifiers (Ryoto, Mitsubishi-Kasei Food Corporation, Tokyo, Japan) were evaluated alone or blended with other sucrose esters or with other carbohydrate fatty acid polyesters, and their surfactant properties were examined in terms of their ability to reduce surface and interfacial tensions at different concentrations and to stabilize oil-in-water (o/w) and water-in-oil (w/o) food emulsions at room and refrigeration temperatures, respectively. In general, when used alone the carbohydrate polyesters were excellent stabilizers of w/o emulsions and poor stabilizers of o/w emulsions. Blending lipophilic carbohydrate polyesters with hydrophilic commercial emulsifiers, such as S-1670, produced stable o/w emulsions and unstable w/o emulsions. Our results suggest that emulsifier blends of potential fat substitutes with FDA-approved commercial sugar ester emulsifiers can be prepared for possible use in low-calorie foods, cosmetics and pharmaceuticals as o/w and w/o emulsifiers.     

Kinetics of crosslinking of linear polyethylene with t-butyl peroxide

The kinetics of the curing of polyethylene with di-t-butyl peroxide have been studied using a Wallace-Shawbury curometer and a Monsanto rheometer. Results obtained for the activation energy of the crosslinking reaction are of comparative value only, because the temperature of the specimen is not known with sufficient accuracy; this applies especially to the rheometer. It was not possible to crosslink polyethylene premixed with carbon black; the addition of carbon black to the mixture of polyethylene and peroxide greatly reduced the degree of crosslinking obtained.