Andrew Klein

Molecular parameters and their relation to the adhesive performance of acrylic pressure-sensitive adhesives

Model acrylic pressure-sensitive adhesives (PSAs) based on poly(2-ethyl-hexyl acrylate-stat-acrylic acid) and poly(n-butyl acrylate-stat-acrylic acid) at 97.5/2.5 wt % were synthesized using semicontinuous emulsion and solution polymerizations. Microgels formed in the lattices retained their discrete network morphology in the film. In contrast, acrylic solution was essentially gel free and crosslinking in the film was provided by the reaction of acrylic acid and post added Al Acetyl Acetonate after solvent evaporation, which led to continuous network morphology. The difference in film network morphology caused significantly lower shear holding power for the film from emulsion PSA compared with that of solvent-borne film. Unlike shear holding power, loop tack and peel of acrylic PSAs were mainly controlled by the same sol/gel molecular parameters, regardless of emulsion or solution PSAs. The important molecular parameters are sol-to-gel ratio, entanglement molecular weight, weight average molecular weight, and to a lesser extent, glass transition temperature.

Dynamic modeling and state estimation for an emulsion copolymerization reactor

Abstract The development of a nonlinear dynamic model of the seeded semicontinuous emulsion copolymerization of vinyl-acetate- n -butyl acrylate for copolymer composition control is summarized. Because the available process measurements do not yield the entire state vector directly, nonlinear state estimation is used. Process disturbances and measurement errors are modeled as stochastic processes and a hybrid extended Kalman filter is employed for state estimation. The filter is based on the local linearization of the process model around the suboptimal filter estimates which are needed for the effective composition control of the produced copolymer.

Details of the emulsion polymerization of styrene using a reaction calorimeter

An automated reaction calorimeter was used to directly monitor the rate of emulsion polymerization of styrene using different emulsifier (sodium lauryl sulfate) and initiator (potassium persulfate) concentrations. By using this technique in conjunction with off-line measurements of the evolution of the particle size distributions, important details of the process were observed. The classical constant rate period (Interval II) often reported for the batch emulsion polymerization of styrene was not seen in this work. Instead, the experimental results suggest that the end of nucleation and the disappearance of monomer droplets take place at approximately the same conversion (36-40%). From the polymerization rate data, important parameters such as the monomer concentration in the polymer particles and the average number of radicals per particle were calculated.

An experimental study of adaptive Kalman filtering in emulsion copolymerization

Abstract Monitoring and control of emulsion polymerization reactors presents several difficulties because of profound modeling limitations and on-line measurement problems. The use of Kalman filtering to obtain optimal estimates of the process states in the presence of modeling inaccuracies, process disturbances and limited measurements is investigated. Different filter designs were developed and experimentally tested. Adaptive filtering was found to be extremely useful in tracking time-varying model parameters of large uncertainty and more robust in the case of unknown process noise statistics. The model used in all Kalman filter designs describes accurately the polymerization kinetics but grossly oversimplifies the thermodynamic relationships for the monomer distribution between the aqueous and polymer phases as well as the time dependency of the average number of radicals per particle. Despite this apparent process/model mismatch acceptable estimates of the process states were obtained.

Molecular parameters and their relation to the adhesive performance of emulsion acrylic pressure‐sensitive adhesives. II. Effect of crosslinking

Acrylic emulsion pressure-sensitive adhesive (PSA) films generally have much lower shear holding power than that of their solvent-borne counterparts for the same peel and tack. This is due to their discrete microgel morphology in the film. In contrast, film cast from solution-polymerized acrylic PSA forms a continuous network as a result of crosslinking acrylic acid and aluminum acetyl acetonate (AAA) in the film following the solvent evaporation. Novel acrylic emulsion PSA was made by copolymerizing ≤1 wt % isobutoxy methyl acrylamide (IBMA) in the polymer backbone. The IBMA grafted the linear portion of the acrylic polymer with the microgels upon heating the film, which resulted in a significant increase in the shear holding power.

Mechanistic studies in tackified acrylic emulsion pressure sensitive adhesives

Twenty-three wt % aqueous tackifier dispersion based on glycerol ester abietic acid (Tg 5 64°C, Mw 5 940) was added to emulsion polymer 50/32/15/3 poly(2- ethyl hexyl acrylate-co-vinyl acetate-co-dioctyl maleate-co-acrylic acid) pressure sensi- tive adhesive (PSA). From these latices, 25 mm thick films were cast. The films were dried at 25°C for 24 h or at 121°C for 5 min. Dynamic mechanical analysis (DMA) of the films included measuring elastic modulus (G9) and damping factor (tan d). Under the above drying conditions, the films did not produce significant differences in their DMA and PSA properties as measured by loop tack, peel, and shear holding power. DMA of the tackified acrylic film showed thermodynamic miscibility between the tackifier and polymer regardless of the drying conditions. Microgels formed during emulsion poly- merization of the acrylic PSA brought inherent weakness to the tackified film proper- ties. In the neat acrylic PSA film, these discrete networks entangled with the un- crosslinked chains while in the tackified film, these networks could not form entangle- ments due to the increased molecular weight between entanglements for the uncrosslinked chains. This lack of network entanglements caused shear holding power of the tackified acrylic PSA film to be 43 lower than that of the neat acrylic PSA film.

Miniemulsion polymerization of styrene using the oil-soluble initiator AMBN

The Mettler RC1 calorimeter was used to measure the rate of polymerization of conventional emulsion, homogenized emulsion, and miniemulsion polymerizations of styrene initiated with 2,2′-azobis(2-methylbutyronitrile). It was noted that the rate of polymerization significantly increased as the surface area of the monomer droplets increased. This was taken as strong evidence that in the miniemulsion and homogenized emulsion polymerizations, the fraction of the initiator soluble in the oil phase was responsible for single radical generation. The partitioning of AMBN at 70 °C was measured by high-pressure liquid chromatography to be 134 parts in the oil : 1 part in the water. Predissolving polystyrene in the miniemulsion prior to homogenization resulted in an enhancement in the rate of polymerization, although to a lesser extent than what has been previously noted for parallel miniemulsion polymerizations initiated with potassium persulfate. It was also noted that the method of addition of the oil-soluble initiator (either predissolved in the monomer prior to homogenization or dissolved in a small separate phase of monomer and added directly to the reactor) has a measurable effect on the kinetics in the miniemulsion polymerization of styrene.

Role of the nonionic surfactant Triton X‐405 in emulsion polymerization. I. Homopolymerization of styrene

The emulsion polymerization of styrene was studied using the nonionic surfactant Triton X-405 (octylphenoxy polyethoxy ethanol). Two separate nucleation periods were noted in these polymerizations resulting in bimodal final latex particle size distributions. The partitioning of the surfactant between the phases was found to play the major role in determining the nucleation mechanism(s) in these polymerizations. Although the total concentration of the emulsifier was always added at a level above its critical micelle concentration (CMC) based on the water phase in the recipe, it was found that the portion of the surfactant initially present in the aqueous phase was below its CMC due to the partitioning. This CMC was also found to increase with increasing total surfactant because the distribution of the surfactant (varying ethylene oxide chain length) depended on the partitioning between the phases. Under these conditions, the first of the two nucleation periods was attributed to homogeneous nucleation, while the second was attributed to micellar nucleation.

Emulsion polymerization of styrene using reaction calorimeter. III. Effect of initial monomer/water ratio

The batch emulsion polymerization of styrene was studied as a function of the initial monomer charge. The emulsifier (sodium lauryl sulfate) concentration was fixed at a level in excess of its critical micelle concentration (cmc). The mechanism of the polymerization was examined through data obtained on the rate of polymerization (through reaction calorimetry), the evolution of the number of particles (through the particle size distribution), and the free emulsifier in the aqueous phase (through a mass balance and surface tension measurements). At low monomer/water ratios (final solids content ≤ 10%) nucleation took place throughout the entire reaction, with micelles existing throughout; the rate of polymerization increased sharply to a maximum and decreased just as sharply with no constant rate period. At the higher monomer levels (final solids content ≥ 20%), the increase in rate was initially as sharp as at the lower levels of monomer; however, this was followed by a slower increase in rate brought about by the nucleation of particles in the absence of micelles (homogeneous nucleation); the length of this nucleation increased with increasing monomer charge. A rate maximum was reached at a similar conversion in each experiment, signaling the disappearance of monomer droplets; however, this did not necessarily indicate the cessation of nucleation. All polymerizations proceeded in a similar fashion until the disappearance of the monomer droplets. Based on these and previous results, a modified description of the emulsion polymerization process is proposed. This description rests on the main characteristic of Interval II, which is that it cannot be characteristically represented by a constant rate of reaction and a constant number of particles for emulsifier concentrations above the cmc, contrary to what is generally considered valid; instead it is represented by an increasing rate of polymerization and a continued particle formation. The particles formed during this period, referred to as Stage II to differentiate it from the classical Interval II, are produced by homogeneous nucleation, and the end of this stage is marked by the disappearance of monomer droplets. Particle nucleation may or may not end at this time. In many cases, the shape of the rate of polymerization curve provides by itself the main characteristics of the polymerization process, namely: the end of micellar nucleation, the end of homogeneous nucleation, monomer droplet disappearance, decreasing monomer concentration in the polymer particles, and onset of the gel effect. However, supplementary information, particularly the evolution of the number of particles, is essential to monitor the nucleation process.

Role of the nonionic surfactant Triton X-405 in emulsion polymerization. III. Copolymerization of styrene and n-butyl acrylate

Emulsion copolymerizations of styrene and n-butyl acrylate were conducted at 70°C using varying amounts of Triton X-405 (octyl phenoxy polyethoxyethanol) as emulsifier. The kinetic behavior was found to vary widely depending on the emulsifier concentration. Unimodal particle size distributions were produced at the lowest (4.2 mM) and the highest levels (12.5 mM, 16.2 mM) of emulsifier while at intermediate levels (6.3 mM and 8.4 mM) bimodal distributions were produced; these were reflected in the reaction kinetics. These results were attributed to the surfactant partitioning behavior in the system which led to homogeneous/coagulative nucleation at the lowest level to homogeneous/coagulative nucleation followed by micellar nucleation at the intermediate levels to micellar nucleation at the highest levels. Although the added surfactant levels were all well above that required to exceed its CMC in the aqueous phase, the substantial partitioning into the oil phase lead to conditions well below the CMC of the portion of the surfactant present in the aqueous phase for the lowest and the intermediate levels of the Triton X-405. Consequences of this partitioning were seen in a semibatch reaction where all the surfactant was initially present in the aqueous phase; in this case, too many particle were nucleated leading to massive coagulation due to insufficient surfactant being available to stabilize the particles.