Nip gap

Kumar et al. have studied the miUing behavior of brominated isobutylene-co-p-methyl styrene (BIMS) and its blends with EPDM [40]. Using the theoretical model of Tokita, they have tried to optimize the rmU parameters. They measured the critical nip-gap at which a front-to-back roll transition takes place. They concluded that addition of different fillers reduced the critical nip-gap for BIMS as compared to the gum polymer. The critical nip-gap is lower for systems containing... 

As shown in Fig. 19, polymer adheres to one of the two counter-rotating rolls passing through an adjustable nip gap which creates intense shear. The rolls are temperature controlled and may run at differential speeds to influence further shear intensity. A degree of lateral cross-mixing must also be imposed to ensure overall compositional uniformity of the batch. A rolling bank of polymer located above the rolls provides some additional mixing capability. 

Most roll mills have only a fixed speed and a fixed friction ratio. Consequently, the actual process variables are the roll temperature and the nip gap. By nip gap adjustment one can increase the work input in the blend. The roll mill process capabilities (+) and limitations (—) can be summarized as follows ... 

In the presence of compatibilizers, a similar process might be followed. Kumari et alP mixed poly(ethylene-co-vinyl acetate) (EVA) with NR and NBR (34% ACN content). Blending took place in a two-roll mill at a nip gap of 1.3 mm and at a friction ratio of 1 1.4. The crosslink agent used was DCP at 2 phr and the EVA amount in the mixture did not exceed 6 phr. Kader et alP used an internal mixer to compatibilize with /raw -polyoctylene rubber (TOR) a Standard Malaysian Rubber with NBR (34% ACN content) at a 50/50 weight ratio. The TOR amount reached up to 40 wt% of the blend where the cure package was based on sulfur. ZnO and stearic acid were introduced in the internal mixer while sulfur and N-tert-butyl-2-benzothiazole sulfenamide (TBBS) were added on a two-roll mill. 

Controlling the nip gap the smaller the nip gap, the higher the level of applied shear in the nip and the better the dispersive mixing obtained. 

A web of molten plastic is pulled from the die into the nip between the top and middle roUs. At the nip, there is a very small rolling bank of melt. Pressure between the roUs is adjusted to produce sheet of the proper thickness and surface appearance. The necessary amount of pressure depends on the viscosity. For a given width, thickness depends on the balance between extmder output rate and the take-off rate of the pull roUs. A change in either the extmder screw speed or the puU-roU speed affects thickness. A constant thickness across the sheet requires a constant thickness of melt from the die. The die is equipped with bolts for adjusting the die-gap opening and with an adjustable choker bar or dam located inside the die a few centimeters behind the die opening. The choker bar restricts flow in the center of the die, helping to maintain a uniform flow rate across the entire die width. 

In contrast to most extmsion processes, extmsion coating involves a hot melt, ca 340°C. The thin web cools rapidly between the die and nip even at high linear rates. Both mechanical and chemical bonding to substrates are involved. Mechanical locking of resin around fibers contributes to the resin s adhesion to paper. Some oxidation of the melt takes place in the air gap, thereby providing sites for chemical bonding to aluminum foil. Excessive oxidation causes poor heat-sealing characteristics. 

Heated two roll mills consist of two large metal cylinders mounted closely in parallel, which rotate toward one another, creating a narrow gap between the two rolls. A diagram of a two roll mill can be seen in Fig. 22.7. To heat the surfaces of the rolls, steam, hot water, or oil circulates through the rolls. Preblended material is dropped into the nip in small quantities. 

The nip width (or gap between the rolls) is critical. Increasing the width decreases the shear stress developed and less dispersion occurs. 

Notice that the pressure profile is as described previously that is, it rises along the x direction as the melt proceeds throngh the nip area, and it reaches a maximnm before the minimum gap clearance, then drops back to atmospheric pressure at the exit. 

The prepreg is prepared as the carbon paper from a roll is run at approximately 3 mmin-1 through a bath containing the resin solution. Then the impregnated paper is run through a pair of driven stainless steel nip rolls set at a gap of from 0.5 mm to remove excess resin solution and achieve the desired pick-up of 78-80% by weight of the resin solution. 

Toothed rolls such as those of Table 12.8(b) can handle relatively large lumps, for example, 14 in. maximum with 24 in. rolls according to the table. To smooth rolls, the feed size is limited by the angle of nip which depends on the surface conditions but often is approximately 16° or arccos 0.961. Accordingly the relation between the diameters of the roll dr and feed df and the gap d0 between the rolls is given by... 

Figure 6.28 Comparison of theoretical and experimental pressure profiles [15], The experiments were performed by Bergen and Scott [2] with roll diameters of 10 in, a gap in the nip region of 0.025 in and a speed U =5 in/s. The measured viscosity was 3.2 x 109 P (Poise, 1 P=0.1 Pa-s).

Fig. 6.22 The nip region of the two-roll geometry, with radii R. A rectangular coordinate system is placed at the midplane in the gap between the rolls connecting the two roll centers.

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