Resin cloud point

A melamine laminating resin used to saturate the print and overlay papers of a typical decorative laminate might contain two moles of formaldehyde for each mole of melamine. In order to inhibit crystallization of methylo1 melamines, the reaction is continued until about one-fourth of the reaction product has been converted to low molecular weight polymer. A simple deterrnination of free formaldehyde may be used to foUow the first stage of the reaction, and the build-up of polymer in the reaction mixture may be followed by cloud-point dilution or viscosity tests. 

Solubility of resins can be predicted in a similar way as for the solubility of polychloroprene rubbers in a solvent mixture (see Section 5.5) by means of solubility diagrams (plots of the hydrogen bonding index (y) against the solubility parameter (5). Another more simple way to determine the solubility of resins is the determination of the cloud point, the aniline and the mixed aniline points. 

Cloud point. Measures the solubility/compatibility of a resin with solvents. The value reported is the temperature at which a specific mixture of a resin and a solvent or solvents blend gives a cloudy appearance, having been cooled from a temperature at which the mixture was clear. Commonly, a test tube of a given diameter is used and the temperature is noted when the lower end of the thermometer, placed at the bottom of the tube, disappears. Resins with cloud points below 0°C are commonly regarded as soluble and cloud points greater than 10°C indicate poor solubility/compatibility. White spirit with various aromatic contents is a widely used solvent in the determination of cloud point, but other solvents or solvents mixtures are also used. 

Aniline and mixed aniline point (DIN 51 775 modified). It is similar to the cloud point test except that the solvent is aniline, a very polar liquid. The aniline point is defined as the temperature at which a mixture of equal parts of aniline and the resin show the beginning of phase separation (i.e. the onset of clouding). Phase separation for aromatic resins occurs between I5°C and below zero for resins with intermediate aromaticity, it lies between 30 and 50°C and for non-aromatic resins, it is 50 to 100°C. Sometimes the mixed aniline point is used. It is similar to the aniline point except that the solvent is a mixture of one part of aniline and one part of w-heptane. The problem of both procedures is that precipitation of resins can be produced before the cloud is generated. 

A more quantitative estimation of compatibility can be obtained with the solvent cloud point test. The solvent cloud point is based on the idea that resins will be compatible with elastomers of similar chemical nature. Thus aliphatic resins will be effective tackifiers for aliphatic elastomers, such as natural rubber, while aromatic solvents are needed for aromatic elastomers, such as SBR. Solvent cloud point tests are carried out in three solvent systems which represent aliphatic, aromatic, or polar systems [16j ... 

Rosin esters show low cloud points and would have wide compatibility with most elastomers. Aliphatic hydrocarbon resins, however, will only be compatible with aliphatic elastomers (e.g. natural rubber). 

The chemical nature of the tackifier also affects the compatibility of resin-elastomer blends. For polychloroprene (a polar elastomer) higher tack is obtained with a polar resin (PF blend in Fig. 27) than with a non-polar resin (PA blend in Fig. 27). Further, the adhesion of resin-elastomer blends also decreases by increasing the aromatic content of the resin [29]. Fig. 28 shows a decrease in T-peel strength of styrene-butadiene rubber/polychloroprene-hydrocarbon resin blends by increasing the MMAP cloud point. Because the higher the MMAP... 

Fig. 28. Evolution of the T-peel strength of roughened styrene-butadiene rubber/polychloro-prene-aromatic hydrocarbon resin blends as a function of the MMAP cloud point. Peeling rate = 10 cm/tnin.

The cloud point curves of the epoxy monomer/PEI blend and BPACY monomer/PEI blend exhibited an upper critical solution temperature (UCST) behavior, whereas partially cured epoxy/PEI blend and BPACY/PEI blend showed bimodal UCST curves with two critical compositions, ft is attributed to the fact that, at lower conversion, thermoset resin has a bimodal distribution of molecular weight in which unreacted thermoset monomer and partially reacted thermoset dimer or trimer exist simultaneously. The rubber/epoxy systems that shows bimodal UCST behavior have been reported in previous papers [40,46]. Figure 3.7 shows the cloud point curve of epoxy/PEI system. With the increase in conversion (molecular weight) of epoxy resin, the bimodal UCST curve shifts to higher temperature region. 

Fig. 13 is a TTT cure diagram of three systems a neat epoxy resin and the same epoxy modified with two reactive rubbers at the same concentration level. The times to the cloud point, gelation and vitrification are shown for each system. The cloud point is the point of incipient phase separation, as detected by light transmission. The modified system with the longer times to the cloud point and gelation, and the greater depression of Tg, contains the more compatible of the two rubbers. The difference in compatibility could then be used to account for differences in the volume fractions of the phase separated rubber-rich domains and in the mechanical properties of the neat and the two rubber-modified systems. 

For high temperature and rubber-modified epoxy resins, thermal degradation events and the cloud point curve are included on the diagrams, respectively. Two degradation events have been assigned devitrification, or a glass-to-rubber event and revitrification, which is associated with char formation. The cloud points and depressions of Tg for different rubber-modified epoxies can be compared and related to volume fractions of the second phase and to the mechanical properties of the cured materials. 

Sensitivity of the Techniques with Respect to Phase Separation. Simultaneous cloud-point and viscoelastic measurements were made at 130 °C. Figure 3 shows that phase separation is not detected at the same time by these two techniques. Complex viscosity is more sensitive to the beginning of phase separation. Cloud-point sensitivity is a function of the wavelength of the light. For our experiment, the use of white light determines the sensitivity at approximately 0.1 xm. However, in the last step of phase separation (when the transmitted light intensity still varies), the increase of viscosity due to gelation of the resin hides the end of the phenomenon. 

The low molecular weight hydrocarbon resins have solubility parameter values in the range of 8.2 to 9.5. This might seem narrow, but the solubility behavior of the various resins is quite different and parameter values to the second decimal point are required in choosing a formulation. Usually a solubility test in specific solvents and a cloud point determination are needed for precise control. 

The interaction between plasticizer and resins must be carefully controlled. Many of the indene resins are controlled within two degrees in the cloud point range. From the relationship... 

More recently, somewhat incompatible systems with a-methylstyrene copolymers, ethylene-vinylacetate resins, and paraffin wax (13) have shown exceptional hot tack although the blend is clearly incompatible. Table III shows that the adhesion increases with the cloud point of the blend. 

Fig. 14. Cloud-point temperature vs volume fraction of CO in a mixture with a stoichiometric DGEBA-EDA (ethylene diamine) system before reactioa The arrow indicates the critical-point composition predicted considering DGEBA-EDA as one pseudocomponent The dotted line represents the branch rich in CO predicted using the pseudocomponent approximation (Reprinted from Polymer International, 30, R.A. Ruseckaite, R.JJ. Williams, Castor-oil-modified epoxy resins as model systems of rubber-modified thermosets. I Thermodynamic analysis of the phase separation, 11-16, Copyright (1993), with kind permission from the Sodety of Chemical Industry, Londoa UK)...

Chen and Jan [133] showed that bimodal distributions could be obtained by using two different rubbers as modifiers of a DGEBA-based epoxy resin cured with piperidine. The rubbers were two acrylonitrile-butadiene copolymers (CTBNs), with different AN content, i.e. 18 and 26%. The miscibility with the epoxy resin (and the corresponding cloud-point conversion) increased with the AN content. Therefore, when 10 wt% of CTBN (26% AN) was used as modifier, a high concentration (Cp == 13.4 pm ) of small particles (D = 0.2 pm) was obtained. When the same amount of CTBN (18% AN) was used as modifier. 

Kauri-butanol value, KB is used for evaluation of dissolving ability of hydrocarbon solvents. It is obtained by titration of a standard Kauri resin solution (20 wt% in 1 -butanol) with the solvent until a cloud point is reached (for example, when it becomes impossible to read a text through the solution). The amount of the solvent used for titration is taken as KB value. The relationship between KB and solubility parameter, 5, fits the following empirical dependence ... 

Melamine-formaldehyde (MF) resins of a molar ratio F/M = 1.70 were prepared at 95°C by dissolving 505 g melamine in 592 g formalin (34.5 wt% aqueous formaldehyde with a pH of 9.2). The reaction was stopped when the reaction mixture reached the cloud point [75]. At 25°C, the pH of the MF resin was adjusted to 7.5 and 9.5. These resins were spray-dried using a Buchi spray dryer and further dried for half an hour in a vacuum oven at 60° C before each MTDSC experiment. Liquid C-NMR spectra showed that more methylene bridges and ether bridges and fewer residual methylol groups (see section 2.2.1 were present in MF pH 7.5 compared to MF pH 9.5. 

Figure 2.16 Unsaturated polyester molecular weight effects on the experimental T versus composition cloud point curves of a UPE resin. Study on resins of similar chemical structure and fonr molecnlar weights (620,1205,1700 and 2740 g/ mole) indicated that UPE miscibility in styrene changed in the following order 1700>1205>2740>620. Reprodnced with permission from F. Baffa and J. Borrajo, Journal of Applied Polymer Science, 2006,102, 6064 2006,...

Different procedures based on that chemistry and using FIA [183], SIA [166,184], MSFIA [185,186], and MCFIA [187] have been described. Most of them make use of online sample preparation, including micellar-assisted preconcentration (cloud point extraction (CPE)) or solid-phase extraction (SPE) [185,186,188]. To this end, either a microcolumn packed with Amberlite XAD-4 resin [185] or C18-modified silica material may be used. 

---