Production of Hot Melt Pressure Sensitive Adhesives
Hot Melt Pressure Sensitive Adhesives (HMPSAs) are composed of Styrenic Block Copolymers (SBC), tackifier, mineral oil, a small amount of antioxidant, and other special additives – such as filler, colorant, and low molecular weight polymer if needed. Most of these ingredients are thermoplastic materials and require a heated environment to mix them together.
Many different types of mixers are available for mixing HMPSAs. Following are two commonly used techniques.
Vertical Reactor mixer. Use of Vertical Reactor is the most economic technique for making HMPSAs. Low molecular weight ingredients, oil and/or tackifiers, must be charged first for this type of mixer. Otherwise, the high torque generated from SBC may damage agitators. Commonly used charging sequence is as follows: 1) mineral oil and antioxidant, 2) tackifiers, and 3) SBC. A good vertical mixer should provide the following features: 1) excellent jacketed hot oil circulation to provide quick heat exchange, 2) adequate agitating configuration to reduce mixing time, 3) scrappers to remove adhesives off the wall and bottom of mixing vessel. This will improve heat exchange and minimize char formation, 4) vacuuming system to avoid oxidation during mixing.
Horizontal Sigma mixer (Sigma blade mixer or Kneader). This type of mixer is normally accompanied with an extruder for easy adhesive discharging purpose, named mix-truder. The mixing sequence of a horizontal mixer is just opposite to that of a vertical one. Generally, the sequence is as follow: 1) SBC plus antioxidant, 2) tackifiers, and 3) mineral oil. It is more efficient to produce a batch of HMPSA with a horizontal mixer versus a vertical one due to its faster heat exchange and higher shearing torque. Additionally, a horizontal mixer can handle more viscous materials with its heavy duty rotor. It is important to point out that tackifiers must be added slowly, portion by portion, after the SBC is fully masticated. Otherwise, the molten tackifier will function like a lubricant to the SBC. This will greatly reduce the shearing efficiency of rotor and result in a poor mixing. Since, the mixing sequence starts with SBC, if the mixing chamber is not under a vacuum environment, the SBC tends to oxidize with surrounded air or oxygen disregarding the addition of antioxidant.
In summary, the vertical mixers offer the following two advantages:
The system is much easier to pull vacuum during mixing. HMPSAs made by this technique normally offer better aging performances compared to those made with a horizontal mixer.
The material charging sequence is more flexible. There is no need for a skillful labor working on a vertical mixer.
The disadvantages of vertical mixers are:
It is somewhat difficult to produce very high viscosity products due to its lower shearing torque.
The heat exchange is somewhat slow. The total mixing time is therefore longer than those made with horizontal mixers.
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