Crosslinking studies of polyether–ester-based polyurethane systems

Ether- and ester-based prepolymeric diols suitable for polyurethane elastomer formation were prepared from polyoxytetramethyleneglycol and polyoxytetramethylene–oxypropyleneglycol with dicarboxylic acids like adipic acid, maleicacid, and phthalic acid. Crosslinked polyurethane elastomers were prepared from these prepolymers using various concentrations of trimethylolpropane as a crosslinker and toluenediisocyanate as the curing agent. Crosslinked polyurethanes were also prepared by a method taking excess toluenedisocyanate other than that of the stochiometric requirement and curing it at a comparatively higher temperature. The data on mechanical properties of these elastomers show that both methods of crosslinking in polyurethane imparts a similar effect on mechanical properties.     

Three‐group type mechanism in the curing behavior of polyacrylate and blocked toluene diisocyanate

Abstract: The curing behavior of two resins, five blocked toluene diisocyanate (TDI) crosslinkers and polyacrylate, was studied using Torsional Braid Analysis (TBA) to examine their dynamic mechanical properties. The curing process was clearly divided into two stages. Associated with data from in situ FTIR at the molecular level, the first stage was the reaction of deblocked NCO and hydroxyl in polyacrylate. In the second stage, the deblocked NCO attacked the hydrogen in NH group of the urethane bond to conduct deep curing. Computer modeling was used to investigate the lowest‐energy configuration of the crosslinker molecule. The space hindrance effect in the molecule causes this type of three‐group crosslinking curing mechanism in the blocked TDI crosslinker. © 2002 John Wiley & Sons, Inc. J Appl Polym Sci 83: 112–120, 2002     

Self‐crosslinkable lignin/epoxidized natural rubber composites

Self‐crosslinkable lignin/epoxidized natural rubber composites (SLEs) were prepared through a high‐temperature dynamic heat treatment procedure followed by a postcuring process. Because of the ring‐opening reaction between lignin and epoxidized natural rubber (ENR), lignin as a crosslinker and reinforcing filler was uniformly dispersed into the ENR matrix and was highly compatible with the polymer matrix; this was confirmed by scanning electron microscopy. The curing behavior, mechanical properties, and dynamic mechanical properties of the SLEs were studied. The results show that the crosslinking degree, glass‐transition temperature, modulus, and tensile properties of the SLEs substantially increased with the addition of lignin. A physical model was used to verify the strong interactions between lignin and ENR. Stress–strain curves and X‐ray diffraction suggested that the reinforcement effect on the SLEs mainly originated from lignin itself rather than from strain‐induced crystallization. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41166.     

Speciality polymer blends of polyurethane elastomers and chlorinated polyethylene rubber (peroxide cure)

Two elastomers having polar reactive functional groups may react with each other upon heating. Considering this view, blends of polyurethane (AU) and chlorinated polyethylene (CPE) elastomer have been prepared where better performance properties can be obtained through interchain crosslinking reaction. The entire study was carried out using dicumyl peroxide (DCP) as a curing agent where both the phases were vulcanized to form new materials. The state of cure gradually increased with the addition of CPE. Hardness, modulus and tensile strength were also increased with increase of CPE content. The elongation at break decreased with higher amount of CPE. After ageing, hardness increased but the modulus and tensile strength decreased. There was drastic change in elongation at break on ageing. IR spectra suggested that interchain crosslinking occurred between AU and CPE elastomers. High temperature DSC studies also revealed the improvement of thermal stability with the addition of CPE. SEM study also suggested interphase crosslinking. © 2000 Society of Chemical Industry     

A New Route to Highly Stretchable and Soft Inorganic–Organic Hybrid Elastomers Using Polydimethylsiloxane as Crosslinker of Epoxidized Natural Rubber

Sulfur or peroxide crosslinking is the most common and conventional method to develop elastomeric materials. A new approach to crosslink epoxidized natural rubber (ENR) by aminopropyl terminated polydimethylsiloxane (AT-PDMS) is described, intending to develop a new kind of hybrid organic–inorganic elastomers. The curing reaction is accelerated by using hydroquinone as a catalyst. The formation of the hybrid structure is evident from the appearance of two glass transition temperatures, at −1 and −120 °C, for the ENR and PDMS phases, respectively. The curing reaction is found to be of first order with respect to amine concentration with the estimated activation energy of ≈62 kJ mol⁻¹. Comparing the mechanical properties to a typical ENR-sulfur system leads to the conclusion that the ENR/AT-PDMS hybrid structure is highly stretchable and soft, as demonstrated by its relatively higher strain at failure (up to ≈630%), and lower hardness and modulus values. The higher stretchability and soft nature of the material are achieved by introducing flexible PDMS chains during the curing process resulting to a hybrid elastomer networks. This kind of soft but robust materials can find several applications in diverse fields, such as soft robotics, flexible, and stretchable electronics.     

A new supertough epoxy formulation with high crosslinking density

A highly crosslinked thermosetting epoxy resin was modified by a reactive blending process carried out in the presence of bisphenol-A polycarbonate (PC). FTIR spectroscopy investigations demonstrated that the presence of PC in the blend decreases the reactants conversion after the curing and postcuring processes. Moreover, it was found that the fracture toughness of this blend system increases markedly by increasing the PC content in the blend. No evidence of phase separation of the minor component during the crosslinking reaction steps was observed.     

IPN中聚氨酯固化反应动力学的FTIR研究

用傅立叶变换红外光谱(FTIR)研究纯聚氨酯弹性体和聚氨酯/聚二甲基硅氧烷IPN中聚氨酯的固化反应动力学。结果表明,在PU/PDMS IPN体系中聚氩酯的交联反应仍为二级反应,聚二甲基硅氧烷的存在大大降低了PU/PDMS IPN的交联速率,并提高了反应活化能。     

UV exposure and temperature effects on curing mechanisms in thin linseed oil films: Spectroscopic and chromatographic studies

The influence of thermal and photochemical treatment in the curing mechanisms of thin linseed oil films is studied. The role of copper acetate pigments is also investigated under the above conditions. UV and FTIR absorption spectroscopy and chromatographic product analysis are used as the main analytical techniques, which allowed the identification of significant changes in the curing mechanism in each case. Yellowing, crosslinking, and fragmentation inside the film material proceed to a different extent according to the conditions. Peroxide destruction is induced by UV exposure at early curing stages and affects the course of the yellowing process. Yellow product formation is also favored by curing at elevated temperatures, which is accompanied in this case by decreased crosslinking in the cured film. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 936–949, 2002; DOI 10.1002/app.10117     

Crosslinking effect on polydimethylsiloxane elastic modulus measured by custom‐built compression instrument

A macroscopic compression test utilizing a simple custom‐built instrument was employed to measure polydimethylsiloxane (PDMS) elastic modulus. PDMS samples with varying crosslinking density were prepared with the elastomer base to the curing agent ratio ranging from 5 : 1 to 33 : 1. The PDMS network elastic modulus varied linearly with the amount of crosslinker, ranging from 0.57 MPa to 3.7 MPa for the samples tested. PDMS elastic modulus in MPa can be expressed as 20 MPa/PDMS base to curing agent ratio. This article describes a simple method for measuring elastic properties of soft polymeric materials. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41050.     

Influence of rubber on the curing kinetics of DGEBA epoxy and the effect on the morphology and hardness of the composites

The influence of the end groups of two liquid rubbers on curing kinetics, morphology, and hardness behavior of diglycidyl ether of bisphenol-A based epoxy resin (DGEBA) has been studied. The rubbers are silyl-dihydroxy terminated (PDMS-co-DPS-OH) and silyl-diglycidyl ether terminated (PDMS-DGE). Crosslinking reactions, investigated by shear rheometry, ranged 90–110 °C, using a constant concentration (5 phr) of liquid rubbers and 1,2-Diamino cyclohexane (1,2-DCH) as hardener agent. The gel time, t _gel, of the neat epoxy significantly decreased when adding the elastomers, more so for the silyl-dihydroxy terminated elastomer; at 110 °C the reaction was nearly complete before rheological test started. The results suggest that the elastomers induced a catalytic effect on the curing reaction. Scanning electron microscopy revealed phase separation of the elastomer during the curing reaction with rubber domains about 5 μm size. However, the DGEBA/dihydroxy terminated elastomer composite cured at 110 °C exhibited a homogenous morphology, that is, the rapid reaction time would not allow for phase separation. Water contact angle tests evidenced either more hydrophilic (silyl-diglycidyl ether terminated rubber) or more hydrophobic (silyl-dihydroxy terminated rubber) behavior than the neat epoxy. The latter effect is attributed to the presence of aromatic rings in the backbone structure of PDMS-co-DPS-OH. Microindentation measurements show that the elastomers significantly reduced the hardness of the epoxy resin, the DGEBA/ether terminated composite exhibiting the lowest hardness values. Moreover, hardness increased as reaction temperature did, correlating with a reduction of microdomains size thus enabling the tuning of mechanical properties with reaction temperature.     

Formation and crosslinking of latex films through the reaction of acetoacetoxy groups with diamines under ambient conditions

We investigated the processes of film formation, polymer diffusion, and crosslinking of latex films at ambient temperature, using low T_g methacrylate latex bearing acetoacetoxy groups, and curing the systems with 1,6-hexanediamine as the crosslinker. The addition of diamine induces floc formation, which modifies the rheological properties of the dispersion and increases its drying rate when coated onto a substrate. The crosslinking reaction between diamine and acetoacetoxy groups occurs at a rapid rate, even in the dispersed state. Although the crosslinking reaction precedes polymer diffusion in the two systems we examined, latex films with relatively good solvent resistance are obtained. Department of Chemistry, Toronto, Ontario, Canada M5S 3H6. Department of Polymer Chemistry, P. O. Box 513, 5600 MB Eindhoven, The Netherlands.     

Isocyanate crosslinking in two‐component waterborne coatings

Two‐component isocyanate curing films (2C NCO) yield a unique combination of properties, such as excellent mechanical and chemical resistances and very good film formation. For waterborne 2C NCO curing applications, dedicated polyols and polyisocyanates have been developed. Film properties such as drying rate, hardness development and conversion of isocyanate groups were found to depend strongly on the hydroxyl group content of the polyol, and with that the amount of polyisocyanate crosslinker which is added to a formulation. Tack‐free time of a film does not, however, correlate to the conversion of isocyanate groups and, hence, to crosslink density. The crosslink density and chemical resistances increase with hydroxyl group content, although not all chemical resistances increase (coffee resistance) along this trend. This is explained with hydrolysis of isocyanates resulting in more hydrophilic urea links. Due to early vitrification of the curing film, diffusion of crosslinker and, hence, mixing of polyol and crosslinker are impaired. As a result, with increasing hydroxyl group concentration, an increasing part of the crosslinker cannot react with hydroxyl groups and can only undergo hydrolysis. © 2018 Society of Chemical Industry     

动态硫化NBR/CPE/EVA热塑性弹性体的研制

为了获得一种耐热油，弹性好，强度高，橡胶感强，易加工的油封材料，我们采用动态硫化法制备了共混型NBR／CPE／EVA热塑性弹性体，系统讨论了橡塑并用比，硫化温度和硫化时间对热塑性弹性体的凝胶含量及力学性能的影响。结果表明：在一定温度和时间范围内，硫化温度升高和硫化时间延长，热塑性弹性体的凝胶含量增大，随着热塑性弹性体中EVA用量增大，弹性体的拉伸强度，赵氏硬度和拉伸永久变形有增大。     

Improvement in compression performance of the polysulfide sealant by thiol‐acrylate reaction

Trimethylolpropanetriacrylate (TMPTA) was added to the polysulfide‐manganese dioxide (PSF‐MnO₂) liquid mixtures as a crosslinker to improve their crosslinking capability. The samples were cured at room temperature for different times and the crosslinking degree was characterized by extraction and swelling tests. Mechanical properties of the cured samples including tensile, compression (stress relaxation, permanent set, and cyclic compression), and dynamic mechanical behaviors were investigated. The results indicated that the TMPTA crosslinker significantly increased the crosslinking degree and the homogeneity of the formed PSF networks. As a result, the tensile and compression stress and relaxation performances of the cured PSF rubber were dramatically improved. This result was also consistent with the results from the swelling, cyclic compression, and dynamic mechanical measurements. Interestingly, the tensile strength of the TMPTA cured samples did not show apparent change when the curing time was longer than 14 days, whereas their compression stress and relaxation performance were growing remarkably from 14 to 60 days. The improved performances were attributed to the high efficiency of thiol‐acrylate Michael addition reaction for the crosslinking. It promoted the curing rate, resulting in good compression properties in a much shorter curing time.POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

Crosslinked acrylic pressure‐sensitive adhesives. I. Effect of the crosslinking reaction on the peel strength

For pressure‐sensitive adhesives (PSAs) composed of poly(co‐ethyl acrylate‐2‐ethylhexyl acrylate‐2‐hydroxyethyl methacrylate) as a base resin and polyisocyanate as a crosslinker, the relationship between the crosslinking reaction and peel strength was investigated. A 90° peel test of cured PSA films under various storage conditions was carried out. At the same time, the isocyanate (NCO) consumption in these PSA films was monitored by attenuated total reflectance/Fourier transform infrared spectroscopy. The peel strength of the PSA compounded with the crosslinker decreased as the NCO groups were consumed. The elevation of the aging temperature promoted the crosslinking reaction and increased the decrement in the peel strength. The peel strength of noncrosslinked and crosslinked PSA films increased with the contact time. A high storage temperature made the increment in the peel strength increase. The addition of the crosslinker to the PSA films reduced the increment in the peel strength. Furthermore, PSA films with residual NCO groups possessed stronger peel strengths than fully cured films. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1493–1499, 2003     

Stepwise increase of hardness in the curing process of one-step phenolic folding compounds

The curing behavior of one-step phenolic molding compounds was studied using results of Rockwell hardness measurements at elevated temperatures with a cone indenter. The curing process was found to advance in a stepwise manner to the final stage of cure at a specific temperature. An explanation for this phenomenon has been attempted by suggesting the alternating occurrence of two types of curing reactions—propagation and crosslinking. After proceeding to some extent of cure, only the propagation reaction can occur through the migration of low molecular resin to the reactive sites of polymer chains. The propagation facilitates the subsequent crosslinking which drastically enhances the hardness of the resin. The results of electrical resistivity measurements and acetone extraction tests were also found to support the occurrence of above phenomenon.     

Novel polymethoxylsiloxane‐based crosslinking reagent and its in‐situ improvement for thermal and mechanical properties of siloxane elastomer

Polymethoxylsiloxane (PMOS) with dense pendant silicone‐methoxy groups was synthesized from cyclosiloxane monomers by ring‐opening polymerization and dehydrocoupling reaction. Synthesis reactions were followed by IR spectroscopy and ²⁹Si NMR analyses. PMOS was used as crosslinking reagent for room temperature vulcanized polydimethylsiloxane (PDMS), and the apparent activation energy for crosslink reaction was 3.92 kJ/mol. TEM study shows that many dispersed high crosslink density PMOS phases were formed in siloxane elastomer as well as the PDMS networks, and the crosslink density increased from PDMS networks to PMOS phases gradually, without a clear interface. It was detected that these PMOS phases improved the thermal and mechanical properties of siloxane elastomer significantly because of their in‐situ microscale improvement effect. TG analysis demonstrated that thermal decomposition process of PMOS crosslinked siloxane elastomer was divided into three stages, the second one corresponding to a possible loss of some new structures, and the residual mass at 500°C was 66 wt %. The crosslink density went up as the loading of PMOS increased. Tensile stress and elastic module increased twice and three times, respectively, when the PMOS content increased from 15.1 to 41.6 wt %. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

The dependence of the rate of crosslinking in poly(dimethyl siloxane) on the thickness of coatings

We consider reasons why the crosslinking reaction rate in poly(dimethyl siloxane) (PDMS) network coatings might differ from the rate found in the bulk and specifically examine the influence of coating thickness. Infrared spectroscopic ellipsometry (IRSE) is employed as an in situ probe of the reactions between vinyl (-CHCH₂) end groups on PDMS and SiH groups in a crosslinker and between unreacted SiH groups and hydroxyl/silanol groups within PDMS coatings, all on silicon substrates. Measurements of the concentrations of SiH groups (using the characteristic vibration at 2160 cm⁻¹) were obtained from coatings between 1 and 27 μm in thickness, over temperatures ranging from 25 to 120 °C. First-order kinetics are exhibited in the consumption of SiH groups. The reaction rate constant is found to decrease with increasing coating thickness. Although there is evidence that the Pt catalyst segregates to the interface with the substrate, this phenomenon does not appear to have an impact on the thickness dependence. The diffusion of water into the silicone might be the rate-limiting step in the reactions, however, and therefore lead to the observed thickness dependence of the reaction rate.     

Ionic elastomers based on carboxylated nitrile rubber and magnesium oxide

The crosslinking of carboxylated nitrile rubber (XNBR) with magnesium oxide (MgO) leads to an ionic elastomer with thermoplastic nature and better physical properties than the ones obtained with other metallic oxides. The crosslinking reaction leads to the formation of a metallic salt as unique product, as it could be seen on the ATR analysis of the samples, prepared at different reaction times. The mechanical properties of the material increase with the amount of crosslinking agent and segregation of an ionic microphase takes place. The presence of this microphase is demonstrated by a relaxation at high temperatures. The apparent activation energy of this relaxation is smaller than the activation energy of the glass transition of the elastomer, the former being more dependent on the amount of metallic oxide than is the latter. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1894–1899, 2007     

GAP/PET双软段含能聚氨酯弹性体的性能

为改善聚叠氮缩水甘油醚(GAP)的性能,选用环氧乙烷/四氢呋喃共聚醚(PET),以三羟甲基丙烷(TMP)为交联剂、异佛尔酮二异氰酸酯(IPDI)为固化剂,制备了GAP/PET/TMP/IPDI双软段含能聚氨酯弹性体,采用FTIR、DSC、XRD等手段对弹性体进行了表征。实验结果表明:在催化剂含量为0.1%时,两种软段与IPDI的反应速率变化有一定差异,但能够共同形成交联网络结构;PET软段的引入使得弹性体拉伸强度提高0.9MPa,延伸率提高156%;弹性体显示出两个软段的Tg,并随两类软段含量的不同而变化。所制弹性体为非晶聚合物,183℃开始分解。