These white powders look unremarkable, but they account for almost 70% of the annual use of inorganic powders. They are the largest and most widely used powder filler in the plastics industry – calcium carbonate, at a low price. Many advantages such as excellent processing properties have become the material of choice for the plastics processing industry. In addition to the plastics field, calcium carbonate is increasingly used in silicone gums. A small amount of nano-calcium carbonate (CCR) is usually added to reinforce and reduce the cost when preparing the silicone gum, and also maintains a good appearance of the colloid. However, nano calcium carbonate needs to pay attention to the following problems during the application process: 1. Water content causes powder agglomeration When the calcium carbonate has a higher moisture content, the hydroxyl group (-OH) on the surface of the particles increases, and the aggregates tend to agglomerate each other, forming a three-dimensional network under the action of liquid polysilane, so that the viscosity of the rubber increases, and in the base material. The formation of 1 to 3 mm particles causes an increase in the mixing time. Therefore, the calcium carbonate powder must be dried before use to control the moisture content below 0.8%. 2, the second agglomeration results in a larger particle size Secondary agglomeration is generally easy to occur in nanometer calcium carbonate products with smaller particle size. As the particle size range of nano calcium carbonate is reduced to 40-60 nm, the specific surface area of ​​the particles increases (22-34 m2/g), and the cohesion is enhanced. It is easy to form a hard group which is tightly bound, that is, a porous secondary particle. During the kneading process of silicone glue, it is difficult to disperse the secondary particles uniformly, and when the number of particles is large, the surface of the product is prone to particles, even "pitting" or "matte" phenomenon. It is therefore necessary to disperse by one or more grindings or to extend the kneading time. 3, the pH is too high, catalytic curing Too high a Ph value will lower the storage stability of the silicone rubber, and the higher the Ph, the faster the silicone adhesive will cure. Storage stability is a very important quality index of silicone rubber products. Theoretically, the pH value of calcium carbonate is weakly alkaline. It can be coated with a weak organic acid or an organic acid salt. A certain neutralization effect, the pH value is controlled below 9.5. 4, insufficient surface treatment or excess When the surface treatment is insufficient, the surface of the calcium carbonate particles is a polar portion, which is difficult to be compatible with non-polar organic substances in the silicone rubber, which causes difficulty in dispersion, and it is difficult to "powder" during kneading to prolong the kneading time even after thorough mixing. Because of the lack of sufficient organic surfactant coating on the surface of calcium carbonate, the interface between the silicone gum system and the polar calcium carbonate is significantly increased, while the surface of the calcium carbonate has more hydroxyl groups, and these groups can be combined with liquid silicone rubber molecules. The Si-O bond in the chain forms a hydrogen bond (physical adsorption), and as a result, two different effects are produced: on the one hand, the physical and mechanical properties of the vulcanizate are improved, and on the other hand, a structuring phenomenon occurs inside the system. This leads to a decrease in the storage stability of the compound. When the surface treatment agent is excessive, the production of silicone rubber is also adversely affected, which may result in a decrease in bonding performance and a decrease in physical properties of the product. Effect on bonding performance: Because silicone rubber is a kind of viscose product, it must be adhered to the surface of the construction medium. In order to improve the adhesion performance, silicone adhesives are more modified with silane coupling agent. The adhesive property is that the reactive groups in the silane coupling agent form a physical bond with the surface of the construction medium by van der Waals force or hydrogen bonding or form a chemical bond by the reaction of the group. When the surface treatment agent of calcium carbonate is excessive, the number of organic groups is significantly increased (especially, the nano calcium carbonate product with the organic hybrid as the main surface treatment agent is more obvious), and some of the groups in the silane coupling agent are combined with carbonic acid. The organic groups in the calcium surfactant molecules are bonded to affect the bonding properties at the construction interface. Impact on the physical properties of the product: The excessive amount of surface treatment agent reduces the probability of direct hydrogen bonding of the surface of the calcium carbonate particles with the silicone gum system, mainly relying on the combination of surfactant organic molecules and the system, because the calcium carbonate surfactant molecules are mainly organic long chain molecules, which The binding force between the organic molecules is relatively flexible, so the cured silicone rubber product has a low modulus. If a proper part of the surface of the calcium carbonate can be hydrogen bonded to the silicone rubber system, the network structure of the system Stronger and more cohesive. The tear strength of such products will increase. In addition, the short-chain organic matter in the surface treatment agent is volatile, and when the treatment is excessive, the volatile matter of the product is increased, so that the low-boiling organic matter extracted during the vacuum kneading of the silicone rubber is increased. 5, affecting the storage stability of the dealcoholized rubber This problem has been seen in some silicone rubber companies, causing greater confusion for nano-calcium carbonate and silicone rubber companies. Due to the production process and product characteristics of the silicone rubber, the finished product obtained by adding the crosslinking agent to the silicone rubber product must be sealed and stored. Once the finished product has quality problems, it is difficult to rework the finished product, resulting in a large loss. According to relevant data, the dealcoholized silicone gum generally uses a highly hydrolyzable reactive silane coupling agent. In the absence of introduction of a hydroxyl group and a moisture scavenger, a trace amount of moisture in the calcium carbonate and the silane coupling agent are easily reacted to form a free alcohol. Thereby causing a decrease in the storage stability and vulcanization performance of the system. In particular, products with insufficient surface treatment absorb moisture very quickly during storage, and it is difficult to exclude the secondary particles of nano-CaCO3. Therefore, it is reasonable to assume that the surface of calcium carbonate particles under this condition has more water and hydroxyl groups. A local microscopic network structure with calcium carbonate as a node is formed. In the severe case, local microstructuralization and stress concentration occur, and more finely distributed "granules" (actual shrinkage or protrusion) are formed. Another peculiar phenomenon of this "particle" is that it gradually disappears when the temperature of the system rises. It can be explained as: due to the increase of the temperature of the system, the thermal motion of the molecule is intensified, the microscopic cross-linking is destroyed, and the local stress is weakened. Or disappeared, so the surface and internal molecular structure of the silicone rubber returned to the normal state, and the temporary "particles" disappeared. When the temperature of the system is lowered, the "particles" reappear in their original positions. Printed Swim Ring,Swim Ring For Kids,Inflatable Pvc Swim Ring,Summer Swim Ring J&TZ CO.,LTD , https://www.inflatable-toy.com
Common problems and solutions of nanometer calcium carbonate in silicone rubber