Moisture during extrusion

Extrusion. The effects of excess moisture during extrusion may include a foamy melt, lower melt viscosity, bubbles in the extrudate, surging that results in arrow heads and wave forms in the extrudate, surface roughness, and reduced mechanical properties. 

The formation of moisture gradients within the material during extrusion, resulting in migration of liquid from within the interparticulate spaces to surface of the extrudate, has been reported (40,25). At low initial moisture content, the friction within the chamber can result in higher temperatures leading to loss of moisture of up to 12% from the extrudate surface. This effect is more pronounced with A-A extruders. 

Use of unmodified starches in combination with mild acids and carbonates has been claimed.214 Starch moisture content should be no greater than 25%. Acids such as tartaric, citric or malic acids are added at levels between 0.2% and 7%, while the carbonate level is 0.1% to 2%, both based on total starch composition. The acid is stated to serve a two-fold function it depolymerizes the starch during extrusion via hydrolysis, improving expansion at the same time, it liberates carbon dioxide, which acts as a blowing agent, from the carbonate. A typical formula extruded in a twin-screw extruder at 170-195°C yielded a product with a density of approximately 1 lb/ft3 (16kg/m3) and resilience of 60-85%. The foam had a continuous skin with a closed cell structure. 

The term dry extruder is applied to a particular type of single-screw machine. These machines were originally designed solely for processing whole soybeans. For this operation, the addition of moisture was not necessary because the high oil content of the beans acted as a lubricant during extrusion. Thus, the machines could reasonably be referred to as dry extruders. ... 

Figure 14. Effect of moisture on protein dispersibility index (PDI) and lipoxygenase enzymes activity (LOX) during extrusion at 99° C.

In an extrusion-spheronization process, formulation components such as fillers, lubricants, and pH modifiers play a critical role in producing pellets with the desired attributes. The granulated mass must be plastic and sufficiently cohesive and self-lubricating during extrusion. During the spheronization step, it is essential that the extrudates break at appropriate length and have sufficient surface moisture to enhance formation of uniform spherical pellets. 

Another important reinforcement application is in silicone rubber. Historically, fumed silicas have played the major role here, but recently precipitated silicas have been developed that possess the characteristics required for this application (6). Compared to conventional precipitated silicas, a product designed for this end use must have higher purity (to impart acceptable electrical properties, because silicone rubbers are often used as insulating materials) and lower water adsorption (to prevent bubbles from forming during extrusion and to impart resistance against moisture pickup). Good dispersibility is also important. 

Since the presence of moisture can result in porosity in processed material, this can be removed by pre-drying the additive before processing, downstream melt devolatilisation, for example during extrusion of profile, or by use of low moisture content filler, available from some manufacturers. 

Recently, mixtures of pectin, starch and glycerol (PSG) were extruded rather than solution cast into films in an effort to reduce the cost of film fabrication (11). SEM images revealed that the temperature profile in the extruder (TP) and the amount of water present during extrusion could control the degree to which starch was gelatinized. TDMA revealed that moisture and TP during extrusion, and by inference that the amount of starch gelatinization had little affect on the mechanical properties of PSG films. TDMA also revealed that extruded films underwent the same thermal transitions as cast PSG films. 

To test this Working Hypothesis, 0%, 0.5%, 1% and 2% cysteine were added to wheat flour as a radical scavenger during extrusion (3). The original Bouncer flour (14% protein) was extruded at process moisture 16% (w/w), constant die temperature 185 C, 225 g/min mass flow rate, and screw speed 500 rpm conditions that provided optimum wheat flour expansion, flavor and textural quality in the earlier experiments. 

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