Mixed Mineral Thixotropes A New Class of Additives for Molded Thermoset Systems

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1 Mixed Mineral Thixotropes A New Class of Additives for Molded Thermoset Systems by Terrence P. Brennan Industry Manager Southern Clay Products, Inc Church Street Gonzales, Texas Presented at a meeting of the Thermoset Resin Formulators Association at the Hilton Suites Chicago/Magnificent Mile in Chicago, Illinois, September 15 through 16, 2008 This paper is presented by invitation of TRFA. It is publicly distributed upon request by the TRFA to assist in the communication of information and viewpoints relevant to the thermoset industry. The paper and its contents have not been reviewed or evaluated by the TRFA and should not be construed as having been adopted or endorsed by the TRFA.

2 Thixotropes are a class of additive that are designed to alter the rheological characteristics of a system. A typical thixotrope will cause the system to thin under shear with a delayed recovery response as shear is released. Mixed Mineral Thixotropes (MMT s) represent a new class of thixotropes. These thixotropes are extremely easy to incorporate and are extremely efficient. Due to their high degree of shear thinning and fast recovery, they also allow formulators to achieve application and performance properties before the onset of appreciable higher shear viscosity MMT s have a very high performance to viscosity ratio. They will compete very effectively against conventional thixotropes such as fumed silica or organoclay. Veteran formulators know that using two or more rheology additives will often generate synergistic performance. With a blend of minerals, MMT s can generate that synergy in a single package. The primary mineral in a MMT is sepiolite. Other proprietary minerals are used to improve dispersion and efficiency. Surface treatments are chosen to compatibilize the MMT with the system in which it will be dispersed. The MMT aggregate is disordered compared to a conventional organoclay. This disordered aggregate allows the product to disperse more readily because it is not necessary to overcome as much surface contact prior to separating platelets.

3 The manifestation of this disordered aggregate that is most easy to readily observe is bulk density. Note that MMT s are much bulkier than fumed silica so they do not dust, yet they are much less bulky than organoclays because of the disordered nature of the aggregate. Initial work with MMT s focused on unsaturated polyester laminating resins and high solids (up to 100%) epoxies. MMT s have also enjoyed success in other resin systems, particularly 100% solids systems or systems that employ aromatic or oxygenated solvents. For maximum efficiency, MMT s should first be dispersed (in order of preference) in solvent, monomer, or the lowest viscosity resin. A Cowles or other high speed disperser (or equivalent) is preferred although it is quite feasible to work with these materials with relatively low shear devices. When effectively dispersed, MMT s will generate 30 to 40% efficiency improvements versus fumed silica and 50% or more versus organoclays. It is important to remember that because of MMT s heightened performance to viscosity ration that your product will now be at a lower viscosity than it was with the previous thixotrope. Formulate to the performance criteria, not the viscosity. Rheological enhancers such as BYK 1 R-605 in unsaturated polyesters or BYK 1 AT-204 in epoxies may further improve the efficiency of MMT s. These additives should be evaluated at 10% of the weight of the MMT. Performance improvements of 60 to 75% are achievable with these additives.

4 usually packaged in 10 kilogram bags. The mechanism by which MMT s is able to achieve these impressive results are centered on the nature of the rheological profile of the thixotrope itself. When compared to conventional thixotropes, the rheological profile of an MMT differs significantly in two important measures. The first is the relative slope of the curve. The slope of the curve for the MMT is much Because MMT s are not efficient viscosifiers (recall the heightened performance to viscosity ratio), it is possible to develop much higher solids in your predispersion phase. In styrene it is possible to manufacture predispersions at up to 15% solids which will remain fluid and are indefinitely stable. As supplied, MMT s are finely divided, off-white powders. They do not appreciably dust and are steeper indicating that it is more shear thinning. Secondly is the recovery rate as indicated by the closeness of the returning curve to the initial curve. These two properties mean that MMT s will generate impressive rheological properties with low high shear viscosities.

5 Both isophthalic resins and di cyclo pentadiene (DCPD) resins benefit unique low viscosity/fast recovery profile. In filled UPR systems, notice the improvement in sag resistance (drainage control) when comparing MMT to fumed silica and organoclays. Perhaps even more notable than the measure of sag resistance is the failure mode as noted on the bottom of each of the sag charts. Notice the total absence of tear drops with the MMT containing system versus either fumed silica or organoclays. This translates to reduced phenomena such as squeeze out.

6 In a separate system, it is seen that with predispersion of the MMT one can achieve 40% or more improvement in performance, or a reduction of 40 to 50% of the amount of thixotrope in the system. In an orthophthalic resin, the unique shear response and recovery the MMT containing system is readily observable compared to fumed silica. Note the lower starting point of the MMT system (corresponding to lower high shear viscosity) followed by the crossing of the curves at about 50 seconds (corresponding to faster recover time). Lower high shear viscosities mean you can raise solids and still maintain your application properties. This translates to lower Volatile Organic Compounds (VOC s)

7 The use of MMT s versus conventional thixotropes presents the formulator with a pleasant choice. He can generate equal performance at a lower load of thixotrope, or substantially improved performance at the same loading level. MMT s also do an excellent job of controlling syneresis (phase separation). MMT containing systems do not exhibit significant post wetting decreases in viscosity.

8 MMT s also perform in a superior manner for controlling settling as noted with the epoxy samples containing quartz sand. In many cases, MMT s will allow waxes and other incompatible items to be held in a homogenous state, not allowing any phasing of the previously incompatible products. This ability to control phasing or separation translates well to finished or molded parts. There is a dramatic reduction in bloom from waxes and other additives when MMT s are used.

9 There is also a much greater ability to maintain homogeneity when using micro balloons or hollow spheres. These products also tend to float in resin. This tendency can be controlled or eliminated with MMT s. MMT s have also been found to dramatically improve color homogeneity in complex geometry parts that have sharp radii. The tendency for the pigment to migrate out of sharp radius areas under flow is almost eliminated by MMT s. MMT s represent an exciting opportunity for the formulator to overcome a wide range of rheological problems by controlling flow while not imparting excessive viscosity. Easily incorporated, MMT s are a unique additive whose functionality and value to the formulator of thermoset resins is limited in large degree only by your imagination.