Study on the Spinning of Polyether Ether Ketone

Polyether ether ketone (PEEK) is a special engineering plastic with excellent comprehensive performance. It was first developed by Imperial Chemical Industries (ICI) in the late 1970s. With the development of thermoplastic resin based composite material forming technology and the expansion of the application range of this material, it is necessary to spin PEEK resin into high-performance special fibers with heat resistance and chemical corrosion resistance.

In 1981, Slater reported the research results of preparing PEEK matrix composites using fiber hybrid method. The key to the fiber blending method is to prepare resin fibers with a diameter equivalent to the reinforcing fibers, and then mix the two fibers into a composite yarn through COMMINGLING or SERVING, weaving it into a prepreg. The prepreg can also be directly woven with both fibers. This method has great appeal because it can not only weave two-dimensional fabrics, but also weave three-dimensional structures with almost no margin, greatly improving the thickness direction strength of the workpiece, material toughness, and damage tolerance, while also shortening the production cycle.

At present, PEEK fibers can be used not only as composite materials for aviation, but also for drying fabrics, processing fabrics, screw conveyor belts, filter screens, high-temperature gas filter felt, as well as high-performance canvas, cable wrapping materials, sports goods, and other production.

 

1. Comparative analysis of various spinning technologies

 

In the past thirty years, synthetic fiber spinning technology has made significant progress. For the same fiber forming material, different spinning technologies can sometimes obtain fiber products with significantly different physical and mechanical properties. Therefore, the fiber industry is committed to improving existing processing processes and developing new processes. However, the development of spinning technology has not been fully explained in a large number of patents.

There are three conventional spinning methods, namely melt spinning, wet spinning, and dry spinning In melt spinning, the bulk polymer melts and is extruded from the spinneret, while the liquid filament solidifies through the cooling medium. Commodity fibers such as nylon, polyester, and polyolefin fibers are melt spun. An important requirement for melt spun polymers is that they do not degrade during heating and softening In wet spinning, the polymer is dissolved in an appropriate solvent, and its solution is extruded from the spinneret to form a solidified silk thread through a coagulation bath. The solvent in the spinning solution is removed through a reverse diffusion mechanism in the coagulation bath In dry spinning, after the polymer solution is extruded from the spinneret, the filament passes through a closed chamber filled with dry hot air, and the solvent evaporates to solidify the filament.

Dry spinning is generally used in situations where polymers have a high melting point or are prone to degradation upon heating. The requirement is that the solvent has a low boiling point and low heat of vaporization, and is easy to recover, has good thermal stability, is inert, non-toxic, is not easy to cause static electricity, and has no explosion risk. Polyacrylonitrile fibers and polyvinyl chloride fibers are dry spun.

From the perspective of solvent recovery, it is advisable to use concentrated solutions for dry spinning. However, due to the limited solubility and difficulty in mastering of polymers, the preparation of many polymer concentrated solutions is actually very difficult. Dry spinning is not suitable for PEEK spinning. Compared with wet spinning, melt spinning has two advantages, namely high yield and solvent-free recovery. The high yield is due to the small resistance of the wire when passing through the air cooling medium. Due to the much higher viscosity of the melt compared to the wet spinning solution, the stretching ratio of the melt spinning is very high. The stretching ratio has a significant impact on the molecular orientation and crystallinity of the finished fiber, so an optimal stretching ratio can be selected to control the physical properties of the finished fiber during melt spinning. In addition, wet spinning is less economically attractive than melt spinning, as removing solvents from the filament requires additional costs. Sometimes, in order to further stretch the wire, some post-processing steps are required.

Melt spinning can be divided into furnace grid spinning method and screw extrusion method. The furnace grid spinning method was invented by the United States in the late 1930s for the spinning of polyamide 66. The main drawback is that the melting ability of the furnace grate varies depending on the viscosity of the melt and is limited. In addition, the interface between the melt and the surrounding gas updates extremely slowly, making the polymer prone to decomposition. Therefore, it is necessary to clean the furnace grate frequently. The screw extrusion method has been increasingly used for the spinning of long and short fibers since the mid-1970s. Its main advantage is that the polymer stays in the melting chamber for a short time, and the interface can be continuously updated due to screw transportation. Therefore, its melting ability is large and its productivity is high.

2. Factors affecting the PEEK melt spinning process

 

2.1.1 Effect of PEEK melt rheological properties

 

When selecting spinning parameters and equipment, it is necessary to consider the rheological properties of polymers. PEEK has a high molecular weight and high melt viscosity at temperatures below 350 ℃, requiring high extrusion pressure during extrusion. Moreover, due to the poor fluidity and obvious viscoelasticity of the melt at this time, the extrusion pressure will sharply increase with the increase of extrusion speed. Technically speaking, the desired viscosity range at the shear rate of the spinneret hole is 10-200Pas. The viscosity of the melt should be adjusted by temperature within an appropriate range. Yu Jianming et al. conducted spinning experiments using a small plunger extruder and found that:[ Γ]= At temperatures above 350 ℃, the extrusion pressure of a 0.80 PEEK significantly decreases, and at this point, PEEK has preliminarily developed melt spinning performance. Under certain extrusion speed conditions, the extrusion pressure continuously decreases with increasing temperature. When the temperature rises above 370 ℃, the trend of extrusion pressure increasing with the increase of extrusion speed becomes gentle, indicating that the melt fluidity is better above this temperature. Finally, it is concluded that within the temperature range of 380-400 ℃, it is more suitable for[ Γ]= Melt spinning of 0.80PEEK. But this conclusion is only applicable to furnace grate plunger spinning, and for screw extrusion spinning, the shear effect of the screw must also be considered.

Most polymer melts are subjected to shear stress, with molecular chains oriented in the direction of shear stress, resulting in a decrease in flow resistance. As the shear rate increases, the viscosity of the melt correspondingly decreases, exhibiting the flow characteristics of a pseudoplastic fluid.

Through rheological experiments on three types of PEEK resins (PEEK450P, PEEK380P, PEEK150P), it was found that with different molecular weights of PEEK, the fluid consistency K and flow behavior index n of the melt are different. As the molecular weight decreases, the melt consistency K decreases, the melt viscosity decreases, and the melt flow behavior index n gradually increases. The degree of deviation of the melt flow behavior from Newtonian fluids decreases, approaching Newtonian fluids. Among the three PEEK resins, 150P has the smallest molecular weight, K value, best flowability, and n value, which is closer to Newtonian fluid.

The experiment on the relationship between the apparent viscosity and shear rate of PEEK resin melt shows that PEEK resin melt is relatively sensitive to shear rate. As the shear rate increases, the apparent viscosity decreases significantly. Obviously, the flow characteristics of PEEK melt can be utilized to improve the melt spinning ability of the screw extrusion spinning process.

 

2.1.2 The influence of crystal structure and cooling conditions on the process

 

PEEK is a crystalline polymer, but only partially crystallized, with crystalline regions composed of grains distributed in homogeneous amorphous regions. According to different thermal histories, crystallinity can vary within a certain range. The performance of PEEK is influenced by its crystalline structure. Firstly, the presence of crystalline regions alters the structure of homogeneous amorphous regions. The crystalline zone is similar to a type of reinforced particle, which can lead to a certain degree of increase in resin strength, modulus, etc., but slightly decrease in toughness; Secondly, the presence of crystalline zones makes the resin prone to shear stress during the molding process, leading to the formation of anisotropic materials and lower transverse properties compared to homogeneous amorphous resins.

Crystalline and amorphous thermoplastic resins have different melting characteristics, and both exhibit a glassy state below Tg. Above Tg, molecular motion intensifies, and the amorphous region changes from a glassy state to a viscoelastic state, allowing the resin to flow plastically. And the crystal structure is destroyed until Tm, before which the crystalline micro zone exists as a rigid region.

Process parameters such as molding temperature, insulation time, and cooling rate also have an impact on crystallinity, with cooling rate having a greater impact. Different cooling rates can cause changes in the crystallinity of PEEK between 0 and 48. But under general molding conditions (cooling rate between tens to hundreds of degrees per minute), the crystallinity is generally around 30. Process parameters not only affect crystallinity, but also have an impact on crystal shape. In PEEK resin, a low cooling rate is conducive to the formation of large spherulites, while rapid cooling often results in small spherulites. However, rapid cooling causes a rapid decrease in the flowability of PEEK after being extruded from the spinneret. PEEK filaments in the stretching flow zone may be in a highly elastic state with poor flowability, resulting in a certain degree of molecular orientation of the primary filaments formed at a certain initial stretching ratio.

From a theoretical analysis, the high orientation of raw silk is unfavorable for having high stretching ratios. In addition, due to the certain degree of crystallization of PEEK raw silk, if stretched at relatively low temperatures, the crystalline structure cannot be fully destroyed during stretching, which may not fully reflect the influence of the orientation degree of raw silk on tensile properties.

 

Therefore, PEEK spinning needs to maintain a high hot channel temperature (above 260 ℃) to prevent the rapid cooling of the filaments extruded from the spinneret, which will help reduce the orientation of the primary filaments and aid in their stretching.

 

2.1.3 Effect of drafting process conditions on the properties of PEEK fibers The glass transition temperature Tg of crystalline polymer PEEK is 144 ℃

[1]. If the drawing is below Tg, it belongs to forced high elastic deformation. The closer the temperature is to Tg, the easier the deformation is to be achieved. If stretched above Tg, the chain segment can move and the crystallization behavior of macromolecules begins to manifest. At this time, the high elastic deformation and crystallization of macromolecule chain segments under external force occur simultaneously. Related literature studies have shown that the maximum drafting ratio of PEEK raw silk increases with the increase of drafting temperature below Tg, reaching

Maximum value. Above Tg, due to the temperature range of 160~180 ℃ being the fastest crystallization rate of PEEK macromolecule segments, the fibers are prone to fracture due to the stress concentration caused by crystallization during stretching. Therefore, its maximum draft ratio has decreased. The stretching temperature continues to rise above 200 ℃, and as the binding of molecular chain segments further decreases, the crystallization of molecular chains weakens, and the maximum stretching ratio begins to increase again, reaching the optimal level

.

The relevant experiments on PEEK drafting wire also indicate that at the maximum drafting ratio, the crystal orientation of PEEK drafting wire monotonically increases with the increase of drafting temperature, and the strength of drafting wire basically increases with the degree of fiber orientation. At a drafting temperature of 190-250 ℃, the orientation of the drafting wire is relatively high, and the mechanical properties of the drafting wire are the best at this time

.

3. Technical issues related to PEEK melt spinning equipment

 

3.1.1 Analysis of exhaust principle and exhaust structure

 

In theory, under stable spinning process conditions, the fiber forming performance of the melt in each hole of the spinneret is the same. Yu Jianming et al

Experimental studies have shown that due to the lack of self exhaust function in the plunger extruder used for spinning, bubbles are easily entrained in the yarn, coupled with fluctuations in the supply of materials to each hole, resulting in individual jet holes experiencing wire breakage first. It can be seen that PEEK melt has high viscosity, difficult flow, and gas contained in the melt and fibers is difficult to eliminate, which is a key technical issue in its spinning and forming process.

During the exhaust process of polymer melt, gas molecules diffuse through both free surfaces and bubbles. Moreover, these two processes coexist, and the formation, expansion, and rupture of bubbles also result from molecular diffusion. The melt in the spiral groove enters the exhaust section at normal or lower pressure through the high-pressure action of the first metering section, and the decrease in pressure promotes the vaporization of water or volatile substances contained in the melt. When the concentration of volatile components in the melt is low, during the exhaust process, the volatile components mainly diffuse in the form of molecules from the free surface; When the concentration of volatile substances contained in the melt is high, foaming exhaust is dominant

.

Peter et al. introduced a device for removing volatile components from high viscosity thermoplastic melts, which mainly consists of a special heat exchanger and a separation container. The heat exchanger is placed vertically, with a conical tube at the upper end for feeding. The feed material is heated and continuously evaporated in the heat exchanger, falling into a thin layer into the separation container. In the separation container, the volatile gas phase is separated from the liquid phase melt, and the exhausted melt is discharged from the discharge port at the bottom of the separation container. The structure of this exhaust heat exchanger is complex, and the entire device occupies a large volume.

Eugene et al

A device suitable for venting high viscosity polymer melts is introduced. It consists of a heat exchanger, vacuum device, stirrer, and discharger. Unlike the literature, a simple tubular heat exchanger is used to heat the feed melt. Due to the addition of an agitator near the discharge port, the exhaust effect is better. This article provides an example of a twin screw extruder using this device to continuously exhaust the grafted rubber polymer.

There are many structural types of exhaust extruders, which can be divided into one level and multiple levels based on the number of exhaust ports; According to the exhaust conditions, it can be divided into natural exhaust and vacuum exhaust. The position of the exhaust port can be located in front of the fusion section or at the end of the fusion section. Designers should conduct structural design based on the performance of raw materials and the requirements of the final product. For powdery raw materials, if the boiling point of the volatile substances contained in the raw materials is relatively low, it is more advantageous to place the exhaust port at the front end of the melting section. For granular materials containing high boiling point volatiles, the exhaust port should be located behind the melting section. For those that require a high limit value of volatile compounds in the extruded product, natural exhaust can be used to prevent material leakage and improve production. However, for those with lower content limits, vacuum exhaust should be used.

 

 

3.1.2 Mechanism and Prevention Measures of Unstable Polymer Melt Flow

 

The melt breakage (MF) during most polymer extrusion is caused by two reasons: one is the surface roughness and unevenness caused by the sliding adhesion phenomenon between the polymer and the capillary wall in the capillary and near the capillary outlet; Another type of melt defect is caused by the phenomenon of melt fracture near the entrance of the capillary tube. Quantitatively describe the difficulty of melt fragmentation, which can be determined by the critical shear stress or critical shear rate value during fragmentation. However, the viscosity of various polymer melts is extremely different, and the critical shear rate for melt fragmentation varies greatly. The occurrence of melt fragmentation depends on the properties of the material and extrusion conditions. As the shear rate increases, the molecular weight of the polymer increases, the melt temperature decreases, the capillary LD decreases, and the inlet angle increases, resulting in melt fragmentation at lower critical shear stresses.

Melt breakage occurs when the extrusion rate exceeds a certain value, while tensile resonance occurs when the elongation ratio reaches a critical value at a certain extrusion rate（ Τ L Τ o) An unstable phenomenon that occurs only after the cr value. As the stretching ratio increases, the frequency of wire diameter fluctuations increases, ultimately leading to wire breakage. The size of the critical stretching ratio is influenced by the geometric shape of the die mouth, processing parameters (temperature, shear rate, cooling conditions, etc.). Experiments were conducted on the relationship between the diameter fluctuation of PP fibers and the apparent tensile ratio at different temperatures. It was found that at 180 ℃, the degree of fiber unevenness increased with the increase of tensile ratio. At 200 ℃ and 220 ℃, the degree of unevenness first increases and then decreases, which means that at very high tensile ratios, even when the critical value is greatly exceeded, very uniform fibers can also be obtained. Therefore, if the melt temperature is selected appropriately, tensile resonance will not occur when the stretching ratio exceeds the critical value.

In addition, the relationship between the geometric shape of the mold mouth and the critical stretching ratio was determined through experiments. The results showed that as the LD of the mold mouth increased, the diameter of the guide hole DR decreased, and the inlet angle decreased, both of which could increase the critical stretching ratio

. From this, it can be concluded that stretching resonance causes uneven diameter of PEEK fibers and increases the possibility of fracture. The method of prevention is to design an appropriate shape of the spinneret hole, reduce extrusion swell, increase the temperature of the melt, and use slow cooling method during cooling.

 

3.1.3 Design instructions for extrusion swelling and spinneret holes

 

During PEEK spinning, there is a phenomenon of extrusion swelling, which is an unfavorable factor for spinning. It limits the fiber's elongation in the plastic state, reduces the effective elongation rate, and affects the refinement of the fiber. When the diameter expansion is severe, it causes the melt to flow over the spinneret plate,

Break the spinning end. If the cooling conditions are not appropriate, the majority of the expansion will not be stretched and brought into the winding wire, which will also cause unevenness of the wire.

Extrusion swelling can be controlled by reasonably designing the geometric shape of the spinneret hole and selecting suitable spinning conditions. The design of spinneret holes mainly involves three important geometric parameters: inlet angle, aspect ratio, and DRD (diameter ratio of guide holes and capillaries) When the inlet angle is conical, the melt gradually deforms, energy is dispersed, and the absorption value is low, not exceeding the critical shear rate value, and melt fragmentation will not occur. The use of a conical inlet angle can also reduce dead space, reduce material retention, and avoid thermal degradation. Melts with high elasticity can use spinnerets with small inlet angles to reduce extrusion swell and increase critical shear rate The aspect ratio affects the pressure drop before and after the spinneret hole and the extrusion swell of the fine flow. A large LD relaxes the entrance effect, reduces extrusion swell, stabilizes the spinning process, and improves fiber quality. But long and thin micropores are expensive to process. The development trend of synthetic fiber micro spinnerets is to increase LD The diameter of the spinneret hole should be determined based on yield, stretching ratio, and fiber size.

The change in capillary diameter can cause changes in many other parameters. Therefore, when the variety and specifications of the polymer have been determined, the pore size should be kept constant and efforts should be made to change the parameters with strong independence.

To obtain uniform fibers, in addition to designing each spinneret hole reasonably, the uniformity of each hole should also be considered for the entire spinneret board. In addition, the machining accuracy of holes is also important, and in order to make the fibers uniform, it is necessary to control the aperture size.

If the size of the spinneret hole with a diameter of 0.25mm and a length of 0.75mm fluctuates by ± 1, the fiber size deviation will reach

 

4. Conclusion

 

(1)[ Γ]= The temperature range of 380-400 ℃ is suitable for PEEK furnace grate plunger spinning with a temperature of 0.80. However, for screw extrusion spinning, the influence of screw shear must also be considered, and the shear effect of the screw can be used to improve the melt spinning ability.

(2) PEEK spinning needs to maintain a high hot channel temperature, adopt a low cooling rate, reduce the orientation of the primary silk, and help with the next step of stretching of the primary silk.

(3) The mechanical properties of PEEK raw silk obtained by hot drawing within the temperature range of 190~250 ℃ are the best.

(4) The gas contained in PEEK melt and fibers is difficult to eliminate, which is a key technical issue in its spinning and forming process. Reasonable and effective exhaust structure design should be carried out based on the performance of raw materials and the requirements of the final product.

(5) The phenomenon of extrusion swelling is an unfavorable factor in the PEEK spinning process, which affects the refinement of fibers and the uniformity of filaments. Melt fragmentation and stretching resonance are unstable phenomena caused by polymer melt flow.

These can be controlled by designing appropriate spinneret hole shapes and selecting appropriate spinning conditions.