IntroductionIn computer science, the acronym GIGO means "garbage in, garbage out", emphasising the need to make sure the information you input is right. It also applies to adhesive formulation: what you get out depends on what you put in, and that reaches beyond ensuring youre using the correct chemical. Thats particularly true in poly(vinyl acetate ethylene) (VAE) water-based adhesives, in which ethylene provides useful formulation versatility. VAE conventionally contains just 5-40 per cent ethylene, in contrast to poly(ethylene vinyl acetate) (EVA) used in hot melt adhesives, which consists of 60-95 per cent ethylene. Incorporating ethylene increases vinyl acetates flexibility, and reduces its glass transition temperature (Tg) making it more flowable and able to form an intimate, strong, bond with a substrate at room temperature.1 Many types of VAE are manufactured on varying scales, and the technology used to do so plays a big part in the emulsions properties. The suspension polymerisation of vinyl acetate and ethylene in water for use in adhesives need not be prohibitively expensive. The longest known, most flexible and least technically demanding approach is to use batch processing.2 By contrast continuous loop processes, despite being more complex, can ultimately be more economical. But, when they were first invented, even the most advanced loop processes were relatively simple.
For example, an early two litre capacity loop reactor consisted mainly of a positive displacement pump and a length of 14 feet of stainless steel tubing in a loop connected to the inlet and outlet of the pump.3 It attached two liquid feeds and an ethylene feed to a gas disperser that fed into the tubing. It also boasted a liquid feed direct through the pump, and used a T-joint fitted with a back pressure regulator to collect the emulsion flowing out. The inventors added enough ethylene through the T-joint to pressurize the system to 1.4 MPa, followed by reagents listed in table 1. After 7 hours of reaction maintained at 20°C by a cooling jacket, the end product was a 24 per cent ethylene VAE emulsion 50 per cent solids by weight. Modern loop reactors can be used with a variety of temperatures and pressures up to 7 MPa, though its usual to operate at 0.3-0.5 MPa, which avoids bubbles forming.4 However they still face some challenges in delivering performance and economy. One common flaw observed is the wall deposits that accumulate along the inside of the tubes.5 Such deposits mean that pump pressure must be increased, and hamper cooling. That can affect the reaction conditions and raise production costs, by demanding either that more energy be put into cooling or that production is slowed down. As deposits build up, process conditions change, undermining the consistent production quality continuous processes should offer. One solution to this is to use cleaning "pigs", solid cylinders the same size as the internal diameter of the reactors tubes. As they are propelled through the loop by the polymerising emulsion, they scrape deposits from the side of the tube. Another issue is grit formation, where unwanted lumps of polymer, about 0.5 to 2 mm in diameter are formed that must be filtered out of the product.6 Grit contents above 0.4% cause manufacturing difficulties because they slow the filter time and generate waste for disposal. Grit formation can be reduced by using high amounts of stabilizer, but this adds cost and is detrimental to ultimate adhesive properties. An alternative approach exploits the order of reagent addition, mixing the monomers and a reductant like sodium metabisulfate into a pre-emulsion that enters the loop reactor through one feed line, and adding oxidants like t-butyl hydroperoxide via a second feed line.
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