Carboxylic latex

Compounding is quite different for the two systems. The solvent base system is dependent on magnesium oxide and a /-butylphenoHc resin in the formulation to provide specific adhesion, tack, and added strength. Neither of these materials have proven useful in latex adhesive formulations due to colloidal incompatibihty. In addition, 2inc oxide slowly reacts with carboxylated latexes and reduces their tack. Zinc oxide is an acceptable additive to anionic latex, however. Other tackifying resins, such as rosin acids and esters, must be used with anionic latexes to provide sufficient tack and open time. 

The carboxylated latexes are formulated to use a reduced amount of a less reactive 2inc complex. Special resin blends provide an optimum balance of film tack and strength, and are coUoidaHy compatible with the carboxylated latexes (158). Epon resins may also be used as an acid acceptor in place of 2inc oxide (160). 

In 1997, a Chinese research group [78] used the colloidal solution of 70-nm-sized carboxylated latex particles as a subphase and spread mixtures of cationic and other surfactants at the air-solution interface. If the pH was sufficiently low (1.5-3.0), the electrostatic interaction between the polar headgroups of the monolayer and the surface groups of the latex particles was strong enough to attract the latex to the surface. A fairly densely packed array of particles could be obtained if a 2 1 mixture of octadecylamine and stearic acid was spread at the interface. The particle films could be transferred onto solid substrates using the LB technique. The structure was studied using transmission electron microscopy. 

FIG. 4 tt-A isotherm of carboxylated latex particles on aqueous subphase, it measured using a WU-helmy balance. (Data taken from Ref. 155.)... 

Wash particles (e.g., 100 mg of 1 pm carboxylated latex beads) into coupling buffer (i.e., 50 mM MES, pH 6.0 or 50 mM sodium phosphate, pH 7.2 buffers with pH values from pH 4.5 -7.5 may be used with success however, as the pH increases the reaction rate will decrease). Suspend the particles in 5 ml coupling buffer. The addition of a dilute detergent solution may be done to increase particle stability (e.g., final concentration of 0.01 percent sodium dodecyl sulfate (SDS)). Avoid the addition of any components containing carboxylates or amines (such as acetate, glycine, Tris, imidazole, etc.). Also, avoid the presence of thiols (e.g., dithiothreitol (DTT), 2-mercaptoethanol, etc.), as these will react with EDC and effectively inactivate it. 

The expansion characteristics of carboxylic latex particles have been measured using three independent techniques sedimentation, which uses the change in particle density due to swelling to determine the change in particle size viscometry, which measures volume changes and photon correlation spectroscopy, which measures the diffusion coefficient of the particles. The sedimentation technique offers precise measurements at low shear but requires relatively... 

Figure 8. Effect of added electrolyte (0.01M NaCl) on the expansion characteristics of a model carboxylic latex as determined by photon correlation spectroscopy (( ) acrylic latex, 2% AA(II), d = 1120 A same latex in 0.01 M NaCl, d0 =...

In addition, maximum expansion occurred at pH = 12.5 in the PCS experiments compared with approximately 10.5 observed in the sedimentation and viscometry experiments. Since PCS is carried out at much lower particle concentrations, interactions between the charged particles at the higher concentrations are probably involved. Similar comparisons with non-expanding carboxylic latex particles were carried out in an effort to separate interparticle charge effects from true particle expansion in interpreting apparent particle sizes determined by hydrodynamic methods. 

The expansion of carboxylic latex 42BRD47 as a function of pH, determined by the sedimentation method, is given in Figure 4. More than a 50% increase in hydrodynamic diameter has been reached at high pH. 

Figure 4. Expansion curve of carboxylic Latex 42BRD47 as determined by the sedimentation method. The estimate of particle diameter by turbidity was 0.43 pm.

Preparation and Characterization of Alkali-Swellable Carboxylated Latexes... 

Preparation of Carboxylated Latexes. Latexes were prepared by emulsion polymerization using recipes given in Table I. Several MMA-MAA copolymer latex samples were prepared with differ-... 

Polymerization Recipe for Preparation of Model Carboxylated Latex... 

The conventional conductometric titration was not suitable for the characterization of these carboxylated latexes because the surface of the latex particles was varied during the titration. In carboxylated latexes, the carboxyl groups located at the particle surface are neutralized and hydrated first, follow ed by the neutralization and hydration of the carboxyl groups located in the inner layers adjacent to the surface, and so on. These sequential reactions seemed to take longer than the experimental time (say, 30 minutes). Verbrugge (4) reported that it took almost two days to get to equilibrium completely. The overall rate of reaction should be controlled by a diffusion process in the neutralization reaction of carboxylated latexes. If this is the case, the rate of reaction must be a function of the distribution of carboxyl groups within the latex particles. 

Figure 3. The 13C NMR spectra of carboxylated latexes (a) MMA-MAA (75 25) batch latex (SPB-2-(5)) (b) MMA-MAA (75 25) semicontinuous latex (SP-30)...

If the swelling behavior of carboxylated latexes could be characterized only by the increase in the volume fraction, i.e., in particle size, Eq. 1 would still hold but this is not the case in carboxylated latexes. The latex particles are swelled to a great extent, and the polymer segments are dissolved into the aqueous phase and interact between the particles by the hydrogen bonds and chain entanglements. Thus, Eq. 1 is not expected to hold at all and the interparticle interactions are expected to be dominant in the viscosity development of the latex. 

The apparent viscosity shows some dependence on shear rate the magnitude of which is a function of pH as shown in Figures 8 and 9 for batch and semicontinuous carboxylated latexes respectively. It is difficult to compare the viscosities of non-Newtonian fluids. One possible method for comparison is to take the apparent viscosity value at a certain shear rate for all samples In this work, the viscosity values at a shear rate of 215 (sec ) were taken to compare the behavior of the samples. The results of viscosity as a function of pH obtained in this manner are shown in Figure 10 for batch latexes, and in Figure 11 for semi-continuous latexes. 

From the above Results and Discussion, one could speculate the following scheme given in Figure 14, of alkali-swelling and/ or dissolving behaviors of the MMA-MAA carboxylated latexes. 

Nishida, S., "Preparation, Characterization and Alkali-Swelling Behavior of Carboxylated Latexes", Ph.D. Dissertation, Lehigh University, 1980. 

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