Matjaž Krajnc

The Structure and Properties of Acrylic-Polyurethane Hybrid Emulsions and Comparison with Physical Blends

Aqueous acrylic-polyurethane hybrid emulsions were prepared by semi- batch emulsion polymerization of a mixture of acrylic monomers (butyl acrylate, methyl methacrylate, and acrylic acid) in the presence of polyurethane dispersion. Equivalent physical blends were prepared by mixing acrylic emulsion and polyurethane dispersion. The weight ratio between acrylic and polyurethane components varied to obtain differ- ent emulsion properties, microphase structure, and mechanical film properties of hy- brid emulsions and physical blends. Particle size and molecular mass measurements, scanning electron microscopy, glass transition temperature, and rheological measure- ments performed characterization of the latex system. The mechanical properties were investigated by measuring tensile strength and Koenig hardness of dried films. The experimental results indicate better acrylic-polyurethane compatibility in hybrid emul- sions than in physical blends, resulting in improved chemical and mechanical proper- ties.

Kinetic modeling of methyl methacrylate free-radical polymerization initiated by tetraphenyl biphosphine

Abstract This study presents a semi-empirical kinetic model for the prediction of monomer conversion and average molecular weight during pseudo-living radical polymerization of methyl methacrylate with tetraphenyl biphosphine as the initiator in bulk, using UV light as the energy source. The model incorporates the chain-transfer to the initiator, which has been shown to be an important reaction for controlling the molecular weight of formed polymers. To solve the set of proposed kinetic equations the method of statistical moments was employed. The kinetic parameters estimated by fitting the initiator conversion data and monomer conversion data were used for modeling the molecular weight profiles, which were in a fair agreement with the experimentally measured average molecular weights. The pseudo-living radical polymerization scheme is characterized by having reversible termination steps. The proposed circular reaction mechanism was lumped in the empirical power-law reaction rate expression, which seemed to be successfully employed in the model development. The modeling results may be used for the characterization of the polymerization process studied as well as for the prediction of the polymerization behavior at monomer conversions up to 30%.

IR Drying of Water-Based Acrylic PSA Adhesives

Results of infrared drying of an acrylic microsphere adhesive coating are presented. The effect of IR drying on adhesive coating was analyzed by Fourier transform infrared spectroscopy (FTIR). The adhesion strength was tested by peel adhesion measurement. Drying was experimentally carried out on materials dried to the same moisture extent at different operating conditions. The power of IR heating source, time of drying, and distance between adhesive coating and heating source were varied. The developed phenomenological model was compared with existing empirical models.

Temperature Dependent Dynamic Mechanical Properties of Hydrogenated Nitrile Butadiene Rubber and the Effect of Peroxide Cross-linkers

Abstract The viscoelastic behavior of hydrogenated nitrile butadiene rubber (HNBR) was studied over a range of temperatures and shear frequencies. Dynamic mechanical properties were studied and modelled using the generalized Maxwell model and the Williams-Landel-Ferry equation. A fitting algorithm was developed to provide the best agreement between the experimental data and the model results. In addition to dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC) was applied. The HNBR structure was characterized by X-ray diffraction (XRD). The developed model exhibited an excellent agreement with either isothermal or dynamic experiment data, yet only up to the rubbery plateau, after which a structure ordering occurred. This was explained by the cyano group secondary bonding and consequentially the cross-linking between HNBR chains. A molecular modeling simulation was made to confirm the cross-linking. The effect of peroxide cross-linking agents in a compound resembled the one usually observed in the filler formulated compounds.

Kinetic investigation of a complex curing of the guaiacol bio-based benzoxazine system

Abstract Curing kinetics of guaiacol based benzoxazine synthesized from guaiacol, furfurylamine and formaldehyde forming bio-based polybenzoxazine was investigated. The curing process showed complex polymerization behavior, as the exothermal signal consisted of several overlapped peaks. Differentiation and fitting of overlapped peaks was performed by Pearson VII distribution obtaining two separate exothermal signals further associated to stage 1 and stage 2. The apparent activation energies of both stages were determined to be 113.8 kJ mol-1 and 117.5 kJ mol-1, respectively, according to Kissinger. The first could be explained by benzoxazine ring-opening and electrophilic substitution, whereas the second stage corresponds to the rearrangement and diffusion-controlled step. Kinetics of each stage was studied separately. As a result, the first stage was described by Šesták-Berggren autocatalytic model, whereas the second stage appeared to follow nth order kinetics proved by the Friedman method. Application of both kinetic models demonstrated that the predicted curves fit well with the non-isothermal DSC thermograms and as such sufficiently describes the complex curing behavior of guaiacol based benzoxazine.

Chapter 5:Acrylic–Clay Nanocomposites by Suspension and Emulsion Polymerization

This chapter is focused on the synthesis and properties of acrylic/clay nanocomposite pressure-sensitive adhesives (PSA), which are an example of use of low glass transition temperature acrylic/clay nanocomposite materials. These acrylic/clay nanocomposites can be synthesized in situ, by carrying out an emulsion or suspension (co)polymerization of a chosen monomer or monomers in the presence of clay. The properties of a PSA depend upon balance of three basic adhesive properties: tack, peel strength, and shear resistance. Hence, to be an effective PSA, the material needs a balance of elasticity and viscosity. Here it is shown how the incorporation of nanosized clay into PSA latex film offers an effective means to modify the viscoelastic properties of PSA, with the aim of optimizing the adhesive properties. The adhesive properties are changed according to the specific end use of the adhesive and can be adjusted by choosing an appropriate nanocomposite synthesis method and by using the proper type and amount of clay.

Modification of NR/BR blend with unsaturated thermo plastic polyurethane; polyurethane synthesis, blend morphology, vulcanization, and vulcanizate properties

Abstract The unsaturated polyurethane (PU) was synthesized, characterized, and used in rubber compound. To study the effect of PU incorporation in the blend a systematic approach was used where PU was added to the natural rubber (NR), butadiene rubber (BR), and NR/BR blends. Different un-vulcanized and vulcanized compounds were characterized and the vulcanization process was investigated. It was observed that the synthesized PU was able to crosslink with the rubber compounds during the vulcanization process. The lowered temperature of melting indicated a better processability of the composites containing thermoplastic PU which melted at processing temperature, thereby allowing improvement in blend viscosity mold flow and to some extent in its homogeneity, although the mechanical properties such as hardness, tensile strength, modulus, and elongation at break did not differ significantly from the properties of elastomer blends without PU. The thermal stability of compounds with PU did not significantly worsen.

Comparison of conventional and controlled bulk polymerization of styrene by N-methyl-2-pyrrolidone and 1-dodecanethiol

Abstract Controlled radical polymerization of styrene was initiated using Nmethyl- 2-pyrrolidone (NM2P) and 1-dodecanethiol (1DT). This polymerization system exhibited “living” characteristics, namely the molecular weight averages of resulting polymers increased linearly with conversion, which has been determined by gel permeation chromatography (GPC) analysis. The polymer was additionally characterized with Fourier transform infrared spectroscopy (FTIR) and proton nuclear magnetic resonance (1H NMR) spectroscopy. Kinetics of controlled radical polymerization has been studied and the temperature dependency of overall polymerization rate constant has been determined. A comparison of conventional bulk radical and controlled bulk radical polymerization of styrene has been explored. Conventional radical polymerization was initiated with 2,2’-azobis(2- methylpropionitrile) (AIBN), whereas 1-dodecanethiol was applied as a chain transfer agent. The reaction rate of controlled radical polymerization was slower than the reaction rate of conventional radical polymerization initiated with AIBN. Consequentially, the activation energy and the pre-exponential factor were lower in the case of controlled polymerization in comparison to the ones observed in the conventional - AIBN initiated - system. Furthermore, by comparing the controlled to the conventional radical polymerization lower polydispersities were observed. Macromolecular structure analysis suggested a more linear chain structure in the controlled radical polymerization system. The thermal properties of polymer products have also been studied and the corresponding glass transition temperatures were higher upon comparison of the conventional and the AIBN initiated system, respectively