Crosslinked latex films

Bradford and Vanderhoff (20) have also prepared films from crosslinked latex particles. These authors studied a 65 35 styrene-butadiene copolymer crosslinked with varying amounts of divinylbenzene and found that although the incorporation of divinylbenzene retarded the coalescence of latex particles, these particles did indeed coalesce, presumably due to a similar interdiffusion of polymer chain ends. 

Pre-crosslinked Latex Blends. In these materials the individual latexes are crosslinked during synthesis, then blended, and a film is formed. Because of limited deformation and/or interdiffusion capabilities, such films tend to be weak, and only used for special purposes [Zosel and Lay, 1993 Lesko and Sperry, 1997], However, light crosslinking, as occurs in SBR latexes, may be tolerated. Pre-crosslinked latex blends materials are actually not IPNs, because the definition requires that at least one of the polymers be polymerized and/or crosslinked in the immediate presence of the other. An application of pre-crosslinked suspension-polymerized blends, in anionic and cationic form, is as ion-exchange resins. In suspensions, the particles are larger, usually of the order of 10-200 pm. 

N-methylol methacrylamide (NMMA) is also water soluble and is typically supplied as a 60% active product in water at a pH of 6.5 [36]. Yeliseeva [39] studied the crosslinking behaviour of latex films prepared from copolymerizing ethyl acrylate and NMMA she found that crosslinking via a hydrogen bonding mechanism can significantly contribute to mechanical strength development in the films. 

Richard at Rh6ne-Poulenc [67] and Zosel at BASF [68] have carried out extensive studies on the dynamic mechanical properties of latex films. They examined latex made from monomers (like butyl acrylate and butadiene) that crosslink spontaneously during emulsion polymerization. Here crosslink density was reduced through the use of chain transfer agents. Alternatively, with monomers like BMA which form non-crosslinked latex, crosslinking was induced through the addition of bifimctiona] monomers such as methallyl methacrylate (MAMA). Richard has recently published a review of his work in this area [75]. 

The efect of crosslinking on the dynamic mechanical properties of polymers has been studied for many years. But one should note a clear distinction in the case of film formation from disposions of crosslinked latex particles. Each particle is a microgel. The crosslinks are confined to the particle itself without any additional ctosslinking across the particle boundaries [68]. Of course in some systems, like those formed from butadiene-containing latexes or from other latexes containing reactive functional groups, cross-boundary crosslinking can occur subsequent to film formation. 

Figure 14.25 Stress-strain plots for samples of PBMA latex films. On the left, top to bottom, samples were annealed >r increasing times at 90 C. On the right, similar films were formed from latex prepared in the presence of a crosslinking agent, 2 mol% MAMA. (Reprinted with permission fi-om ref. [68]. Copyright 1993 American Chemical Society.)...

Figure 18.1 Effect of chain transfer agent, crosslinking monomer and copolymerized MMA on (a) shear modulus and (b) tan S of poly(BA) latex films...

Following a related approach, Castelvetro et al. reported the formation and properties of hybrid latex films resulting from the coalescence of low 7 poly(BA-co-MMA-co-MPTMS) terpolymer latex particles coated by a silica shell [78], The latex was synthesized at neutral pH by semi-continuous emulsion polymerization under starved-feed conditions in order to protect the MPTMS monomer from premature hydrolysis and condensation reactions. A substantial amount of free silanols were therefore available for further reaction with the silica precursor. In order to avoid the formation of a densely crosslinked silica network around the latex core, which may significantly alter film formation, the pH was kept at around 2 (at this pH, hydrolysis is promoted and condensation is significantly retarded). TEM and AFM studies of the nanocomposite film indicated that the silica shell formed a continuous percolating network throughout the polymer matrix. A porous film of interconnected hollow silica spheres was next elaborated by thermo-oxidative decomposition of the organic phase. 

Latex with hydroxyl functionalised cores of a methyl methacrylate/butyl acrylate/2-hydroxyethyl methacrylate copolymer, and carboxyl functionalised shells of a methyl methacrylate/butyl acrylate/methacrylic acid copolymer was prepared by free radical polymerisation. The latex was crosslinked using a cycloaliphatic diepoxide added by three alternative modes with the monomers during synthesis dissolved in the solvent and added after latex preparation and emulsified separately, then added. The latex film properties, including viscoelasticity, hardness, tensile properties, and water adsorption were evaluated as functions of crosslinker addition mode. Latex morphology was studied by transmission electron and atomic force microscopy. Optimum results were achieved by introducing half the epoxide by two-step emulsion polymerisation, the balance being added to the latex either in solution or as an emulsion. 8 refs. 

The use of one-dimensional magnetic resonance imaging(MRI) to provide information on concentration and molecular mobility (as revealed by the spin-spin relaxation time) as a function of depth into crosslinking latex coatings during their film formation was studied. MRI profiles, with a pixel resolution of 9 micrometres, were obtained at regular time intervals from a vinyl... 

Macromolecular Symposia Vol.152, March 2000, p.55-71 CROSSLINKED FILMS FROM EPOXY-FUNCTIONALISED LATEXES RELATIONSHIP BETWEEN CROSSLINKING AND FILM PROPERTIES... 

Journal of Coatings Technology 72, No.903, April 2000, p.45-61 POLYMER DIFFUSION AND MECHANICAL PROPERTIES OF FILMS PREPARED FROM CROSSLINKED LATEX PARTICLES Pinenq P Winnik M A Ernst B Juhue D Toronto.University Elf Atochem... 

The tensile properties results of NR dipped latex films prepared by prevulcanisation process and post-vulcanisadon imply that the standing period during the latex stage dominates the extent of crosslinking in the latex films. Furthermore, CTOsshnking does not sean to occur in dry... 

TOWARDS STUDY OF EFFECT OF CROSSLINKER ON LATEX FILM FORMATION Ghazaly H M Daniels E S Dimonie V L Klein A El-Aasser M S... 

Individual Networks—Poly(urethane-urea). The polymers were self-crosslinking owing to the presence of triols already in the latex. Films were cast on glass with a doctor blade, dried for 15 min at room temperature, 15 min at 80 C (or until completely tack free), and cured for 30 min at 120°C. They were removed from the glass plate by immersion in hot, distilled water. They were dried further at 80°C under a vacuum of 2 Torr. 

To explain the fact that HSPAN swells in water to form gel sheets or macroparticles rather than disintegrating into a gel dispersion, we initially felt that chemical bonding must take place between individual particles of water-swollen gel as water evaporates. Although we cannot totally eliminate this possibility, the proposal of primary chemical bonding is not necessary to explain the behavior of these films and conglomerates. For example, Voyutskii (19) has reviewed the formation of films from vulcanized rubber latexes and concludes that film formation in these systems is observed because of interdiffusion of ends of individual macromolecules in adjacent latex particles. This diffusion can take place even though individual latex particles are crosslinked, 3-dimensional networks and the continuity of the resulting films, even when... 

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