Rate Of Cross-linking Reaction Research Articles

Interdiffusion and crosslinking in thermoset latex films

Thermoset latex systems represent an attractive approach to obtaining the high performance needed in many different kinds of industrial coatings, while satisfying the growing requirement for environmental friendliness. In these coatings in the dispersed state, the reactive groups are packaged inside of polymer particles. These latex particles deform as the coating dries to form a transparent binder phase. The useful properties of mechanical strength, as well as scrub and solvent resistance, develop over time. This paper focuses on the idea that to achieve the desired properties in a thermoset latex coating, one has to pay proper attention to the relative rates of polymer diffusion and crosslinking in the coating. Strength in these films develops as a consequence of chains that connect crosslink points on opposite sides of interface formed between adjacent particles in the film. Thus polymer diffusion must precede extensive bond formation created by the crosslinking chemistry. This paper reviews fundamental concepts and then describes experiments in three separate systems. These experiments show that the formulator has three main strategies to vary the relative rates of these processes: 1. Catalyst strength and concentration will affect the reaction rate. 2. Polymer chain length will affect the polymer diffusion rate. 3. Temperature changes will normally have a larger affect on the polymer diffusion rate than on the crosslinking reaction rate.

An investigation of water crosslinking reactions of silane‐grafted LDPE

AbstractFactors—including time, temperature, morphology, and thickness of sample, the extent of silane grafting, and water concentration—that affect the rate and degree of water crosslinking reactions of the silane‐grafted LDPE are investigated. The gel content of the water‐crosslinked sample increases with increasing time, temperature, and water concentration, but with decreasing content of the crystalline component in the sample and thickness of the sample. The relationship between the gel content and the crosslinking time is dependent on thickness and morphology of the sample, and the extent of silane grafting in the sample. The crosslinking rates and the resultant gel content are inversely proportional to the content of crystalline component of the sample, suggesting that the crosslinking reactions occur mainly in the amorphous domain of the sample. For those samples with high resultant gel contents, the crystallizations of the samples are significantly enhanced by crosslinking when the gel contents are higher than about 40%, leading to a dual relationship between the gel contents of the samples and the crosslinking times. For low temperatures, the rate‐determining step of the crosslinking reactions is the diffusion of water, rather than the hydrolysis and the subsequent condensation reactions of the silyl trimethoxy groups. For high temperatures and high extents of silane grafting in the samples, however, the chemical reactions dominate the crosslinking process. The overall activation energy of the crosslinking reactions is dependent on thickness of the sample. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 186–196, 2001

Silane grafting reactions of LDPE, HDPE, and LLDPE

Vinyl trimethoxysilane and vinyl triethoxysilane grafting reactions, induced by dicumyl peroxide, of LDPE, HDPE, and LLDPE were investigated. The apparent activation energy of vinyl trimethoxysilane grafting reactions was positive when the reactions were induced by 0.2 phr of the peroxide. The apparent activation energies were negative when 0.05, 0.1, 0.15, and 0.25 phr of peroxide were used. The extents of vinyl trimethoxysilane grafting reactions of polyethylenes were in the order of LLDPE > LDPE > HDPE, although the extents of peroxide cross-linking were in the order of LDPE > LLDPE > HDPE. As compared with vinyl trimethoxysilane, vinyl triethoxysilane produced a relatively high extent of grafting reactions of LDPE but showed a relatively low rate of water cross-linking reactions of the silane-grafted LDPE. The investigation of effects of the amount of peroxide on the vinyl trimethoxysilane grafting reaction heats demonstrated that the extent of silane grafting reactions increased proportionally as peroxide was increased until a certain amount (the value was dependent on the amount of silane used). Beyond this amount of peroxide, the silane grafting did not increase, whereas the peroxide cross-linking appeared to increase significantly with increasing amounts of peroxide. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 3404–3411, 1999

Kinetics of the competitive response of receptors immobilised to ion-channels which have been incorporated into a tethered bilayer.

A competitive ion channel switch (ICS) biosensor has been modelled yielding ligand mediated monomer-dimer reaction kinetics of gramicidin (gA) ion-channels within a tethered bilayer lipid membrane. Through employing gramicidin A, functionalized with the water-soluble hapten digoxigenin, it is possible to cross-link gramicidin to antibody fragments tethered at the membrane/aqueous interface. The change in ionic conductivity of the channel dimers may then be used to measure the binding kinetics of hapten-protein interactions at the membrane surface. The approach involves measuring the time dependence of the increase in impedance following the addition of a biotinylated antibody fragment (b-Fab'), which cross-links the functionalized gramicidin monomers in the outer layer of the lipid bilayer to tethered membrane spanning lipid. The subsequent addition of the small molecule digoxin, (M(r) 781 Da), competes with and reverses this interaction. The model provides a quantitative description of the response to both the cross-linking following the addition of the b-Fab' and the competitive displacement of the hapten by a water-soluble small analyte. Good agreement is obtained with independent measures of the cross-linking reaction rates of the gramicidin monomer-dimer and the b-Fab: hapten complex. The rate and amplitude of the competitive response is dependent on concentration and provides a fast and sensitive detection technique. Estimates are made of the concentration of gramicidin monomers in both the inner and outer monolayer leaflets of the membrane. This is used in the calculation of the gramicidin monomer/dimer equilibrium constant, K2D3. Other considerations include the membrane impedance limit set by the membrane leakage which is also a function of the concentration of the gA monomer concentration, and the two-dimensional kinetic association constant k2D2, of the hapten: b-Fab' complex. The gA dimer concentration is dependent on both the concentration of gA-dig and of the tethered streptavidin: b-Fab' complexes. The model shows that the 2D dissociation constant k2D3(-1), must be at least 10 times faster than the 3D dissociation constant k3D2(-1) for digoxin to completely reverse the cross-linked hapten-receptor interaction at the membrane interface.

Probing the conformational states of the SH1-SH2 helix in myosin: a cross-linking approach.

Previous biochemical studies have shown that the SH1 (Cys707) and SH2 (Cys697) groups on rabbit skeletal myosin subfragment 1 (S1) can be cross-linked by using reagents of different cross-linking lengths. In the presence of nucleotide, this cross-linking is accelerated. In the crystal structure of S1, the SH1 and SH2 residues are located on an alpha-helix, 19 A apart. Thus, the cross-linking results could be indicative of helix melting or increased flexibility in the presence of nucleotides. Nucleotide-induced changes in this region were examined in this study by monitoring the cross-linking of SH1 and SH2 on S1 with dimaleimide reagents of spans ranging from 5 to 15 A. A method was devised to directly measure the kinetic effects of nucleotides on the rates of cross-linking reactions. The slow and reagent-insensitive rates of the SH1-SH2 cross-linking in the absence of nucleotides reveal that the equipartitioning of the SH1-SH2 helix among states with different SH1-SH2 separations occurs infrequently. In the presence of MgADP, MgATP, and MgATPgammaS, the rates of SH1 and SH2 cross-linking were increased approximately 2-7-fold for the shortest reagent (5-8 A). Rate accelerations were much greater for the longer reagents (9-15 A): 40-50-fold for MgADP, 25-40-fold for MgATP, and 80-270-fold for MgATPgammaS. To account for any nucleotide-dependent differences in the reactivities of the reagents toward SH2, the rates of monofunctional SH2 modification on SH1-labeled S1 were also measured for each reagent. These experiments showed that the nucleotide-induced increases in the rates of SH2 modification were similar for all of the reagents. Thus, the changes observed in the cross-linking rates are due not only to the type of nucleotide bound in the active site but also to the span of the cross-linking reagent. These findings are interpreted in terms of nucleotide-induced shifts in the equilibria among conformational states of the SH1-SH2 helix.

Molecular weight distribution formed through chain-length-dependent crosslinking reactions

The molecular weight distribution formed through chain-length-dependent crosslinking reactions in free-radical vinyl/divinyl copolymerization is investigated by using Monte Carlo simulations. When the crosslinking reaction rates for larger polymer molecules are reduced, the high molecular weight tails cannot extend smoothly, resulting in distorted, sometimes bimodal distribution profiles, which exhibit qualitative similarity with those reported experimentally. Although the reduced crosslinking reaction rates between larger polymer molecules may not affect the average crosslinking density levels significantly, they can delay the developments of the weight-average molecular weight significantly. Because the wastage of pendant double bonds through cyclization would result in a similar tendency, one cannot distinguish these two types of non-idealities through the measurements of the pendant double bond consumption and the average molecular weight development.

Effects of cyclization and pendant vinyl group reactivity on the swelling behavior of polyacrylamide gels

A series of polyacrylamide (PAAm) gels were prepared by free-radical crosslinking copolymerization of acrylamide and N,N′-methylenebis(acrylamide) (BAAm) in water at various crosslinker (BAAm) and chain transfer agent (isopropyl alcohol, IPA) concentrations. It was shown that only 5 % of the crosslinker used in the feed forms effective crosslinks in the final hydrogels. At BAAm contents as high as 3 mole %, the equilibrium swelling ratio of the gels in water is independent of the crosslinker content in the feed. This is due to the prevailing multiple crosslinking reactions during the gel formation process. At a fixed crosslinker content, the onset of gelation is shifted towards higher conversions and reaction times as the amount of IPA increases. Addition of IPA in the monomer mixture also increases the equilibrium swelling ratio of PAAm gels. It was shown that the gel crosslinking density increases on rising IPA concentration in the feed due to the increasing rate of intermolecular crosslinking reactions.

Interfacial aspects of strength development in poly(methyl methacrylate)‐based latex systems

AbstractFilm formation from poly(methyl methacrylate) (PMMA) latex and PMMA copolymer latex incorporating N‐(iso‐butoxymethyl)acrylamide (IBMA) or methacrylic acid (MAA) has been investigated in terms of the development of tensile strength as a function of annealing time and temperature. Tensile strength is developed through a combination of macromolecular interdiffusion and interfacial crosslinking. The relative rates of interdiffusion vs. crosslinking reactions were studied as a function of temperature and the chemical nature and concentration of the IBMA and MAA functional groups. For low concentrations of these two functional monomers it appears that polymer chain interdiffusion between adjacent latex particles during the film formation process dominates the kinetics of strength development. However, at higher IMBA and MAA concentrations, the higher glass transition temperature at the latex particle surface and intraparticle crosslinking hinders interdiffusion, as reflected by differences in the power law exponent values obtained from the log‐log dependence of tensile strength on annealing time. The power law exponents were higher in the case of PMMA than for both IBMA‐ and MAA‐containing copolymers. There was a greater influence temperature on the tensile behavior for the MAA copolymer system as compared to the IBMA copolymer. In the interfacially crosslinked latex polymer system, there is competition between the interdiffusion and crosslinking mechanisms in determining the final mechanical strength of films during the annealing process. © 1995 John Wiley & Sons, Inc.

Syntheses and crosslinking reactions of self‐curable copolymers containing pendant cyclic iminoethers and carboxylic acid groups

AbstractSoluble copolymers containing both pendant cyclic iminoethers such as 4,4‐dimethyl‐2‐oxazoline or 4,4,6‐trimethyl‐4H‐dihydro‐1,3‐oxazine and carboxylic acid were successfully synthesized by radical copolymerizations of 4,4‐dimethyl‐2‐vinyl‐2‐oxazoline, 4,4‐dimethyl‐2‐isopropenyl‐2‐oxazoline, or 4,4,6‐trimethyl‐2‐vinyl‐4H‐dihydro‐1,3‐oxazine with methacrylic acid and styrene, methyl methacrylate, or ethyl acrylate using AIBN as an initiator in benzene or DMF at 60 or 80°C. The crosslinking reaction of the copolymers obtained did not occur by heating at 70°C. However, these copolymers quantitatively produced gel products by heating at 130°C. The rate of crosslinking reaction of the copolymer increased with increasing pendant cyclic iminoether and carboxylic acid groups. The rate of crosslinking was also affected by the molecular motion of the polymer chain.Our results show that the copolymers of more sterically hindered 2‐vinyl‐2‐oxazolines are more stable and so they can be crosslinked in a controlled manner and at higher temperatures than the previously studied polyoxaziline system.

Unsaturated polyester-styrene resins: Effect of the catalyst concentration on the kinetic rate and activation energy of the radical crosslinking reactions

The T 1 spin lattice relaxation time, measured by pulsed n.m.r., has been used to monitor the effect of the catalyst concentration on the kinetics of the radical crosslinking reaction of unsaturated polyester-styrene resins. The measurements were performed primarily in the intermediate-final part of the reaction under isothermal conditions. The loss of mobility of the protons of the sample has been correlated with the degree of cure and the kinetic constant as well as with the activation energy, and has been evaluated using the semiempirical kinetic expression proposed by Kamal et al. In the concentration range investigated (from 0.05 to 0.3% w/w) for the catalyst cobalt octanoate the decrease of the activation energy for the process was found to be 2.3 kcal mol −1 of double bonds for each 0.1% w/w of cobalt octanoate.

The radiation crosslinking of poly(vinyl chloride) with trimethylolpropanetrimethacrylate. III. Effect of diundecyl phthalate: Chemical kinetics of a three‐component system

AbstractThe radiation chemistry of poly(vinyl chloride) (PVC) blended with trimethylolpropanetrimethacrylate (TMPTMA) and diundecyl phthalate (DUP) has been examined. This three‐component mixture contains a base resin (PVC), a crosslinking sensitizer (TMPTMA), and a physical modifier (DUP). These are the basic components in any radiation‐curable coating. The kinetics and mechanism of the crosslinking reactions were studied with reference to the dependence on radiation dose and blend composition. The polyfunctional TMPTMA underwent polymerization incorporating the PVC into a 3‐dimensional network. DUP remained chemically inert during the irradiation, not being bound to the network. However, DUP by plasticizing the macromolecules and diluting the monomer, changed the kinetics extensively. DUP enhanced TMPTMA homopolymerization, TMPTMA grafting, and PVC crosslinking reaction rates. The effect on the competition between polymerization, grafting, and degradation reactions was examined in terms of enhanced mobility of the reacting species. The influence of these kinetic considerations in selecting a blend composition for a coating application was discussed.

Modification of poly(vinyl chloride). XXXIII. Novel poly(vinyl chloride) foam crosslinked with 6-dibutylamino-1,3,5-triazine-2,4-dithiol

The formulation for producing the PVC foam crosslinked with a novel crosslinking agent such as 6-dibutylamino-1,3,5-triazine-2,4-dithiol (DB) was studied to determine the processing conditions. DB was almost consumed by radical combination with a coexistent blowing agent such as azobisformamide to give a high-density foam with excessively low crosslinking density. Blowing agents such as p-toluenesulfonylhydrazide (TSH) and 4,4′-oxybis(benzenesulfonylhydrazide) (OBSH) gave a crosslinked foam of low density, while a combination of the two crosslinking agents had moderate crosslinking reaction rate. The formulation recommended in the present study consists of PVC Zeon 101EP or Zeon 121 for paste, 100 parts; DOP, 100 parts; DB, 3–5 parts; OBSH or TSH, 10 parts; MgO, 1–3 parts; and RP101 (mixture of Ba, Ca, and Zn stearate), 2 parts, which gave colorless and insoluble foam of apparent density 0.14 under hot pressing at 180°C. The molded foam could be released without extraction of heat from a hot mold for fusion and expansion.

The Effect of Carbon Black Surface Properties and Structure on Rheometer Cure Behavior

Abstract The cure behavior of SBR and NR compounds was examined using the Monsanto Rheometer. The three cure parameters—induction period (td), rate of crosslinking reaction (k2) and maximum increase in torque (ΔL)—were derived from the Rheometer torque curves. The data so obtained were used to calculate Westlinning's αF-values. It was shown that (1) k2 decreases with increasing total acid concentration at the carbon black surface and (2) the value of αF is related to the filler structure and the crosslink density of the rubber matrix. It was also demonstrated that the crosslink density, as measured by ΔL, is influenced by the surface chemistry of carbon black. In SBR stocks the crosslink density increases with increasing total acid concentration, while in NR compounds the reverse is true. These findings were confirmed by examining the tensile properties of regular and oxidized HAF-LS black in SBR and NR compositions. Thus the effect of carbon black surface chemistry on crosslink density can be estimated, for a given recipe, from the value of k2. An empirical relationship between the cure parameters (αF and k2) and filler structure (as measured by extrusion shrinkage) was derived and tested on a wide range of carbon blacks. This relation gives a much better correlation coefficient than the simple relation between extrusion shrinkage and αF alone. Thus by full use of the Rheometer data, the extrusion shrinkage or other structure-related properties of carbon black filled stocks can be predicted.