Gel Spinning

Presented byArka Das

Influence of spinning temperature and spinlinestretching on morphology and properties of Polyethylene gel spun fibre The tensile strength of gel-spun polyethylene fibres hot-

drawn to the maximum draw ratio depends on the spinning conditions such as spinning speed, spinline draw ratio and spinning temperature.

High deformation rates during spinning introduce defects and fibres with poor ultimate properties are produced.

These defects are already present before the hot-drawing step and can be detected indirectly by wide-angle X-ray scattering, since they are accompanied by preferential c-axis orientation parallel to the fibre axis and a shish-kebab structure.

Experimental procedure The linear polyethylene sample used

throughout this study was Hifax 1900 with Mn, = 2 x 10^5kg/mol and Mw = 4 x 10^6kg/mol

5wt% of this polyethylene was dissolved in paraffin oil.

Upon cooling, this solution formed a gel which was fed to the spinning apparatus.

The gel was extruded into a filament at temperatures varying from 170 to 250ºC with a spinning speed of 1 or 100m/min.

Results and discussion Freely extruded

filament have tensile strength same at all temperature.

The tensile strength of fibre drawn in the spinline to a draw ratio of 50 increases considerably with increasing spinning temperature.

Fig. 2 shows clearly that at a given spinning temperature the unfavourable effect of spin line stretching is enhanced at higher draw ratios.

As in the case of a spinning speed of 1 m/min , the effect of spin line stretching diminishes at higher spinning temperatures.

Again the ultimate tensile strengths for different winding speeds merge at a spinning temperature of about 250°C

Results and discussion

Results and discussion

Results and discussion

In Figure 3 maximum filament strength is observed at 100 m/min.

Again the ultimate tensile strengths for different winding speeds merge at a spinning temperature of about 250°C (Fig. 3)

In Fig. 4 this measure of the degree of preferential c-axis orientation parallel to the fibre axis is presented as a function of the spinning temperature for freely extruded fibres and fibres drawn in the spinline to a ratio of 50.

Results and discussion In Fig. 5 the degree of

preferential c-axis orientation parallel to the fibre axis is presented as a function of the winding speed for different spinning temperatures.

The efficiency of creating c-axis orientation parallel to the fibre axis created by a flow-field and trapped by crystallization is determined by the deformation rate (elongation or shear), the exposure time to the orientational action of the flow-field, and the relaxation time.

Results and discussion

In our case this combination shows that the amount of shish-kebab structure increases with increasing preferential c-axis orientation parallel to the fibre axis.

pictures of the inside of the fibre, clearly prove that the whole fibre consists of a shish-kebab structure.

Fig. 9 shows the stress-strain curves of the extracted fibres spun under different spinning conditions.

Fibres containing preferential c-axis orientation parallel to the fibre axis (Curve (a)) have an elongation at break of about 10 to 30% and no yielding is observed.

On the other hand fibres with no preferential c-axis orientation parallel to the fibre break at much larger strains (up to about 900%, Curves (b) and (c)).

The tremendous influence of the presence of preferential c-axis orientation parallel to the fibre axis on the stress-strain behaviour is demonstrated more clearly in Fig. 10.

It shows a very steep drop of the draw ratio at break of the extracted fibres with increasing preferential c-axis orientation parallel to the fibre axis.

Influence of polymer concentration and molecularweight distribution on morphology and properties of Polyethylene gel spun fibre

In addition to spinning temperature, spinline stretching and spinning speed, the properties of gel-spun polyethylene fibres hot-drawn to the maximum draw ratio also depend on the polymer concentration, molecular weight and molecular weight distribution.

Experimental details Two samples of linear polyethylene Hifax 1900

were used, one with a broad molecular weight distribution (Mw = 4 x 10^6 kg/mol , Mw/Mn = 20; referred to as HifaxA) and one with a smaller molecular weight distribution (Mw = 5.5 x 10^6kg/mol, Mw/M n ~ 3; referred to as HifaxB).

1 to 5 wt % polyethylene solutions in paraffin oil were prepared (containing 0.5wt% DBPC anti-oxidant) at 150°C

The gel was extruded into a filament at temperatures varying from 170 to 250°C With an extrusion rate of 1 or 100m/min using a conical die with an exit of 1 mm

Result & Discussion Fig. 1 where the tensile

strength at break for fibres obtained from a 2 and a 5 wt. % gel and spun under various spinning conditions are compared.

However, 2wt % is already close to the minimum concentration at which gel-spinning is still possible.

Fig. 4 presents this ratio for 2 wt % solutions of HifaxA and HifaxB as a function of the spinning temperature for an extrusion rate of 100 m rain-i and a winding speed of 500m/min .

As expected, the degree of preferential c-axis orientation parallel to the fibre axis decreases, with increasing spinning temperature.

• One of the problems encountered during gel-spinning polyethylene solutions is the adsorption of the polyethylene on the wall of the die. This results in fibres with poor properties, because stretching the entanglement network at one end while the other end is anchored by the adsorbed layer on the wall of the die leads to rupturing of the entanglement structure.

• Adding 1 wt. % aluminium stearate to the polyethylene solution.

• A second way to suppress adsorption is to reduce the residence time of the polymer solution in the spinneret below the time required for adsorption.

• Finally, the adsorption can also be suppressed by raising the spinning temperature

In studying gel-spinning of ultra-high molecular weight polyethylene (U~WPE) we came across an interesting effect due to adding 1 wt % of Aluminiumstearate to the spinning solution Whereas without this additive, drawing of the gel directly afterleaving the die, resulted in a rapid decrease in tensile strength of the ultimate fibre, the additive enabled us to pull the extrudate with speeds exceeding 300 m/min while the strength of level was still maintained at about 3 GPa. This unexpected phenomenon induced some further investigations into the influence of additives on the spinnability of polyethylene solutions.

Effects of Additives on Gel-Spinning of Ultra-HighMolecular Weight Polyethylene

Results and Discussions Drawing the gel-

filament composed of only 5 wt % UHMPE in paraffin oil gave rise to a lowering in the tensile strength from 3 GPa at 1 m/min to 0.5 GPa at 25 m/min

Adding 1 wt % of Aluminium-stearate showed no decrease in tensile strength far beyond drawing rates of 70 m/min

Gels containing also I wt % of EPDM rubber,the winding speeds up to 75 m/min have no influence on the tensile strength.

Gel-spun polyacrylonitrile fiber from pregelledspinning solution

Polyacrylonitrile (PAN) fibers have been gel spun from pregelled PAN spinning solution. The pregelled solution had network structure with elevated spinnability, the as-spun fiber from which had more circular cross-section and reduced skin-core difference. Drawing was more effective in inducing the segmental orientation and crystallization in gel-spun fiber than in dry–wet spun fiber. The mechanical properties of the gel-spun fiber were better than those of the dry–wet spun fiber after multi-stage drawing.

Fiber Spinning and Drawing

Certain amounts of PAN copolymers were dissolved in the mixture of 94.8/5.2 (w/w) DMSO/water to produce a viscous PAN/DMSO/water spinning solution with the PAN concentration of 23 wt%.

The solution was deaerated for 3 h at 70°C in a vacuum oven.

In gel spinning, the pregelled spinning solution was prepared by keeping a fresh solution in a thermostatic spinning barrel at 25°C for 2 h.

The pregelled solution was extruded through a spinneret.

Results and DiscussionThe pregelledsolution is more elastic and it is thus less likely to deform during fiber spinning. Hence the resultant fibers are more likely to have circular cross-section.

Surface Morphology and Physical Properties

The pattern of the dry–wet spun fiber (drawn to 60 times) was similar to that of the gel-spun fiber drawn to 45 times, suggesting that the segments in gel-spun fiber were more easily aligned in the direction of drawing.

Dry–wet spun fiber’s tenacity and Young’s modulus were much lower than that of the gel-spun fiber drawn to 90 times and were only comparable to those of the gel-spun fiber drawn to 45 times.

The gel-spun fiber drawn to 90 times exhibited thehighest tenacity, whereas the dry–wet spun fiber drawn to60 times had the lowest tenacity and breaking elongation.

Conclusions Stretching during spinning introduce defects in the

filaments resulting low strength fibre. Gel spinning of polyethylene only leads to fibres with

good ultimate properties if the introduction of defects is prevented. The stresses should be kept below the threshold value for the introduction of defects by avoiding spinline stretching and/or increasing the spinning temperature. If these conditions are not satisfied.

various properties of gel-spun polyethylene fibres depend strongly on the molecular weight distribution and polymer concentration in solution.

Compared with the dry–wet spun fiber, the gel-spun fiber was more likely to possess circular cross-section and homogeneous structure with reduced skin-core difference.

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