A convenient method used to characterize resin compatibility is the determination of cloud points in suitable solvent systems. Cloud point values determine whether the resin is aliphatic, aromatic, or a combination of both; polar or nonpolar; with a high or low molecular weight. This system is only useful for hydrocarbon tackifiers since rosin resins are highly soluble and do not exhibit cloud points at practical temperatures.
Cloud points may be related to compatibility as follows:
Mixed Methycyclohexane-Aniline Point (MMAP) is a measurement of aromatic solubility and determines the aliphatic/aromatic character of the resin using a solvent blend of a 1:2 mixture of methylcyclohexane and aniline. Low MMAP values (<40°C) typify highly aromatic tackifier resins. High MMAP values (>75°C) indicate a resin is fully aliphatic and/or hydrogenated.
Diacetone Alcohol Cloud Point (DACP) determines the polarity of the resin due to the highly polar nature of the solvent system of a 1:1 mixture of xylene and 4-hydroxy-4-methyl-2- pentanone (common name: diacetone alcohol). Since specific adhesion is related to the polarity of a resin, the DACP cloud point can be used as a specific adhesion indicator. A low DACP value, for example, would indicate a polar tackifier with high specific adhesion to polar substrates such as polyester, aluminum, or galvanized steel.
Odorless Mineral Spirit (OMS) determination is only meaningful for aromatic resins dissolved in odorless mineral spirits, a mixture of various aliphatic mineral solvents. Because aliphatic resin types give OMS values lower than -30°C (-22°F), the method is not useful for non-aromatic resins since most laboratories are not equipped to operate at such low temperatures.
The OMS value determines molecular weight and molecular weight distribution. In addition, OMS can determine compatibility characteristics of a resin/polymer system, especially for ethylene-vinyl acetate (EVA) polymers.
For any one generic type of resin, the higher the OMS cloud point, the greater the molecular weight and the molecular weight distribution. In particular, high OMS values can indicate the presence of high molecular weight material (Mz).
Cloud Point Compatibility
Each class of elastomeric polymer commonly used as an adhesive base will have a characteristic range of solubility that defines its region of compatibility with tackifiers. Once these regions have been defined they can be graphically represented as MMAP vs. DACP cloud points for aliphatic resins and MMAP vs. OMS cloud points for aromatic resins. The cloud points of the specific tackifier resins can then be overlaid on the chart to predict the compatibility between the elastomer and tackifier. Cloud points are a practical and efficient way to get an indication of the compatibility between a selected elastomer and tackifier resin; however, the actual compatibility should always be verified by testing such as DMA analysis and adhesive performance.
Figure 1: MMAP and DACP Cloud Point Regions of Hydrocarbon, Terpine, and Rosin-Based Resins
Figure 2: MMAP and DACP Cloud Point Values of Hydrogenated and Aliphatic Resins
Figure 3: MMAP and DACP Cloud Point Values of Aromatic Resins
