Preparation,structure,and properties of melt spun cellulose acetate butyrate fibers

Abstract

The melt spinning of cellulose acetate butyrate (CAB) without any additives is realized according to the thermal and rheological properties of cellulose acetate butyrate raw material. Thermogravimetric analysis reveals that thermal degradation of cellulose acetate butyrate occurs at 275℃ in oxygen. Rheological tests show that cellulose acetate butyrate is a strong shear thinning pseudoplastic fluid. The melt viscosity of cellulose acetate butyrate is found to be relatively sensitive to temperature change and cellulose acetate butyrate melt is difficult to flow until the temperature reaches 230℃. However, thermal degradation of cellulose acetate butyrate during spinning cannot be completely avoided even when the spinning temperature is 230℃. The orientation of cellulose acetate butyrate fibers can be improved by increasing the spinning draw ratio during the spinning process or by hot drawing during the drawing process. Crystallization of cellulose acetate butyrate fibers is facilitated by improving molecular orientation. Owing to the improved orientation and crystallinity, the tensile strength and initial modulus of cellulose acetate butyrate fibers are enhanced. The cellulose acetate butyrate fiber achieves the highest degree of orientation and crystallinity by drawing at 135℃, showing the highest tensile strength at 1.42 cN/dtex. Moreover, dyeing experiments show that the cellulose acetate butyrate fiber can be dyed with a disperse dye and the suitable dyeing temperature is in the range of 80∼90℃.

Keywords

cellulose acetate butyrate mechanical properties melt spinning rheological properties thermal properties

Cellulose esters are derived from the renewable cellulose which is considered as the most abundant biomass in nature,¹ and are widely used as plastics, coatings, optical films, and fibers.² The cellulose ester used to manufacture fibers is primarily cellulose acetate (CA). In addition to applications in textiles, CA is also widely used to prepare cigarette filter tow.^3,4 However, CA is prone to extremely high melt viscosity owing to its intermolecular and intramolecular hydrogen bonds, and is nearly not melt-processable, although it is a thermoplastic.⁵ Therefore, as is well known, commercial CA fiber is manufactured using the solution spinning method which involves the use of large amounts of environmentally unfriendly solvents such as acetone.⁶ The melt spinning method shows many advantages compared to the solution spinning method. For example, no spinning solvent is required for melt spinning, which also means less energy with higher production efficiency. The biggest obstacle for melt spinning of cellulose esters is the narrow processing window between the melt flow temperature and the decomposition temperature due to the relative lability of the polysaccharide backbone at high temperature.² In order to facilitate melt processing of CA, a large quantity of plasticizers must be used in conjunction with CA to decrease the melt flow temperature and improve its poor melt fluidity.^7–13 This method indeed effectively broadened its thermoplastic processing window. However, such a large quantity of plasticizers always significantly reduces the strength of CA. Furthermore, plasticizers with low molecular weight tend to be released into the environment during service of the final products.¹⁴ Hence, health and safety issues may arise due to effusion of the deleterious plasticizers. Some researchers have tried to introduce branches of aliphatic polymers along the polysaccharide chain of cellulose ester.^15–21 Although the grafted polymers provided cellulose ester with improved thermoplasticity, this method is quite complex and costly. Moreover, it involves chemical reaction and greatly changes chemical structures of cellulose esters, resulting in a substantial change in the intrinsic characteristics of cellulose ester. Therefore, it is of great significance to prepare cellulose ester fibers from the pure cellulose ester raw material by melt spinning without any additives. A specific pure commercial cellulose ester raw material that can be melt-spun into fibers has long been expected.

It is well known that the chemical structure, especially the side groups, of cellulose ester exerts a great impact on its properties including the melt processability. CA contains only the acetyl side groups, whereas cellulose acetate butyrate (CAB) contains not only the short acetyl side groups, but also the long butyryl side groups. This implies that CAB presents a much lower degree of molecular regularity compared with CA. The looser and more disordered arrangement of CAB macromolecules provides CAB with much more free volume and elevated molecular mobility. Theoretically, CAB shows a lower viscous flow temperature and better melt processability than CA, and even presents superior thermoplasticity among the numerous common commercial cellulose esters.² Therefore, as one of the most important commercial cellulose esters, pure CAB free of additives is expected to have sufficient melt fluidity to be processed into fibers using the melt spinning technology. Attempts have been made on the melt spinning of pure CAB. Hooshmand et al.²² prepared the CAB fibers composited with cellulose nanocrystals using a twin-screw micro-compounder. However, 15 wt% of triethyl citrate was still used as the plasticizer to facilitate the spinning. Furthermore, the molecular weight of CAB used in the study was very low, resulting in a quite low tensile strength (20.9 MPa). Chen et al.²³ reported the melt spun fibers of immiscible blends of CAB and a cationic dyeable copolyester. Nevertheless, the composition of the CAB component was lower than 50 wt%, and only the fibers produced from the blends comprising less than 10 wt% of CAB could be wound onto the bobbin. No detailed information concerning spinnability and mechanical properties of the pure CAB fibers was provided. In the present work, a specific commercial CAB product (37.0 wt% of butyryl, 13.5 wt% of acetyl, and 1.8 wt % of hydroxyl) from Eastman chemical company was chosen as the raw material and melt-spun into fibers after carefully investigating its thermal and rheological properties. The relationship between structure and mechanical performances of the as-spun fibers as well as the drawn fibers was studied. The dyeability of CAB fibers was also investigated.

Experimental procedure

Materials

CAB (37.0 wt% of butyryl, 13.5 wt% of acetyl and 1.8 wt % of hydroxyl), free of any plasticizer or additive, was supplied by Eastman Co., Ltd. Chloroform (99%), glycerol (98%) and high-pressure liquid chromatography grade tetrahydrofuran were purchased from Sinopharm Chemical Reagent Co., Ltd. and used as received. Dianix Blue ACE supplied by DyStar was chosen for dyeability test.

Melt spinning and fiber drawing

Prior to processing, the CAB powder was dried in a vacuum oven first at 100℃ for 10 h, and then at 120℃ for 2 h before cooling to room temperature. Melt spinning of CAB was carried out using a laboratory-made melt-spinning machine at 230, 240, and 250℃, respectively. The CAB powder was extruded via a single screw extruder (20 mm screw diameter, the length to diameter ratio of 18:1) that was connected to a single-hole spinneret with a diameter of 1.0 mm. The as-spun fibers were then drawn using a fiber-drawing apparatus consisting of a pair of rollers with independent speed controls. Glycerol was used as the hot drawing bath. The fibers were drawn at 135, 145, and 155℃, respectively. The draw ratio was the ratio of the draw-roller speed to the feed-roller speed.

Dyeing of CAB fibers

The CAB as-spun fibers were dyed using an Atlas Linitest Plus laboratory dyeing machine. One gram of a sample was immersed in a 1% o.w.f. of Dianix Blue ACE dyebath of a liquor ratio 100:1. The samples were dyed at 60, 70, 80, 90, and 100℃ for 1 h, respectively. Finally, the dyed samples were squeezed, thoroughly rinsed with cold water, and dried at ambient temperature.

Characterizations

In order to evaluate the crystallinity of the CAB fibers, differential scanning calorimetry (DSC) measurements were performed using a TA-Q20 calorimeter under nitrogen atmosphere, and a pure indium was used as a standard material for temperature calibration of the calorimeter. The CAB fiber samples (3–5 mg) were sealed in an aluminum pan and then heated from 30℃ to 190℃ at a heating rate of 10℃/min. In the DSC thermograms, the temperature at half-height of the corresponding heat capacity jump was defined as glass transition temperature. The peak temperature at the endotherm was considered the melting temperature.

Thermogravimetric (TG) analysis (Netzsch 209 F1) was conducted from 50 to 500℃ with a heating rate of 20℃/min in nitrogen and air atmosphere and flux of 50 ml/min. All samples were well dried before the test.

Capillary rheological properties of CAB were investigated with a two-bore Rosand capillary rheometer (RH2000) using a die with a 0.5 mm diameter, an L/D ratio of 16 and an entrance angle of 120°. The barrel temperature was set at 220, 225, 230, and 235℃. Melt flow index (MFI) was recorded as the mass of molten CAB passing through a fixed capillary under a constant pressure (2.16 kg) at 210, 220, 230, and 240℃ for 10 min (according to ASTM D1238). The average values of the shear viscosity and the MFI based on three measurements were reported.

The surface and cross-section morphology of CAB fibers were observed using an environmental scanning electron microscope (ESEM) (Quanta-250) at an accelerating voltage of 10.0 kV. In order to observe the fracture morphology, the CAB fibers were fractured by immersion in liquid nitrogen for about 2 min. The cross-section and surface of CAB fibers were sputter-coated with a thin layer of gold prior to the analysis to avoid charging.

The molecular weight was measured with gel permeation chromatography (GPC) system (BI-MwA from Waters, LS) equipped with a multi-angle light scattering detector. Tetrahydrofuran was used as the eluent and polystyrene standards served for calibration.

The sonic velocity along the fiber axis was determined by measuring the transmitting time of a sound between two transducers coupled to the specimens. The measurements were performed using a SCY-Ш model fiber sonic velocity apparatus, manufactured by Shanghai Donghua Kaili Technology of New Material Co., Ltd. The birefringence (Δn) and the diameters (D) of fibers were measured using a polarized light microscope (BX-51, Olympus) equipped with a U-CTB Berek compensator. Optical retardation (R) was obtained from the e-line table provided by Olympus Co. under 546 nm monochromatic light. The birefringence was then determined as R/D with the average value of 10 individual measurements.

Two-dimensional wide-angle X-ray diffraction (2D WAXD) patterns of the CAB fibers were collected on a Bruker D8 Discover diffractometer equipped with GADDS as a 2D detector.

Tensile properties of the CAB fibers were investigated using a FAVIMAT single fiber electronic tensile strength tester (Textechno, Germany) at 20℃ and relative humidity of 65%. The initial gauge length and the crosshead speed were 15 mm and 10 mm/min, respectively. Each sample was measured 30 times and the average values were reported.

The dye concentration of the dyebath was determined at the wavelength of maximum absorption (λ_m) before and after the dyeing using a UV/visible scanning spectrophotometer (Perkin Elmer Lambda 35). The dye uptake (D) on the CAB as-spun fibers was calculated by the following equation,

D (%) = \frac{A_{0} - A}{A_{0}} \times 100 %

(1)

where A₀ and A are the absorbance (at λ_m) of the solution before and after dyeing, respectively.

Results and discussion

Thermogravimetric analysis

The thermal stability of CAB raw material was investigated before thermal processing. Figure 1 shows the thermogravimetric and differential thermogravimetric curves of the CAB raw material in inert and oxidative atmospheres. The TG analysis results are summarized in Table 1. As displayed in Figure 1 and Table 1, T_d1%, T_d2%, T_d5%, and T_d10% of the CAB samples measured in nitrogen was approximately 31, 34, 31, and 26℃ higher than those measured in air, respectively. Apparently, oxygen facilitated the thermal decomposition of CAB. Similar results have been reported in literature.²⁴ Since exposure of the CAB melt to air during processing was inevitable and CAB already began to lose weight at 275℃ in air, TG analysis suggested that the spinning temperature of CAB must not exceed 275℃.

Figure 1.

Thermogravimetric and differential thermogravimetric curves of the cellulose acetate butyrate (CAB) raw material in nitrogen and air atmospheres.

Table 1.

Thermogravimetric analysis results of the cellulose acetate butyrate (CAB) raw material.

Atmosphere	T_d1% (℃)	T_d2% (℃)	T_d5% (℃)	T_d10% (℃)	T_dmax (℃)
Nitrogen	306.0	327.0	345.6	356.2	378.8
Air	275.0	292.6	314.5	330.5	372.9

Note: T_d1%, T_d2%, T_d5%, T_d10%, and T_dmax represents the temperature of thermal degradation for 1%, 2%, 5%, 10% weight loss and the temperature at maximum weight-loss rate, respectively.

Rheological properties of CAB raw material

In this work, a melt flow indexer and a capillary rheometer were employed to study the rheological properties of the CAB raw material. As shown in Figure 2(a), MFI of CAB at 210, 220, 230, and 240℃ was measured to be 0.6, 1.4, 4.2, and 8.0 g/10 min. This revealed that the CAB melt was difficult to flow in the melt indexer barrel until the temperature reached 230℃. Therefore, the spinning temperature should not be lower than 230℃ according to the MFI results. Figure 2(b) presents the apparent viscosity of CAB melt (lgη _a ) plotted against the shear rate (lgγ) at the temperature range from 220 to 235℃. Obviously, apparent viscosity decreased with the increase of shear rate at a given temperature, indicating that the CAB melt was a pseudoplastic fluid. At a fixed shear rate, especially at the range of low shear rates, apparent viscosity decreased as temperature increased. For instance, apparent viscosity of CAB melt at a shear rate of 200 s⁻¹ decreased from 319 to 194 Paċs as temperature raised only 15℃ (from 220 to 235℃). As shown in Figure 2(b), the relationship between apparent viscosity and shear rate roughly obeys the power law:

{lg η}_{a} = (n - 1) \lg γ + lgK

(2)

Figure 2.

(a) The melt flow index of cellulose acetate butyrate (CAB) at different temperature. (b) Plots of apparent viscosity of CAB versus shear rate at various temperatures. (c) Plots of apparent viscosity of CAB versus the inverse of temperature of CAB at various shear rate.

where K is the consistency index and n is the non-Newtonian index. By means of the linear regression analysis, n values of CAB melt at 220, 225, 230, and 235℃ were determined to be 0.28, 0.28, 0.27, and 0.28, respectively. These values were far less than 1, suggesting that CAB melt was a fluid with strong shear thinning behavior and its shear viscosity was quite sensitive to variation of shear rate.

The melt flow activation energy (ΔE) can reflect sensitivity of apparent viscosity to temperature. The Arrhenius equation reveals a mathematical relationship between ΔE and temperature (T):

\ln η_{a} = \frac{Δ E}{R} \frac{1}{T} + \ln A

(3)

where R is the gas constant (8.314 J/mol·K) and A is the pre-exponential factor. ΔE can be obtained through the plot of lnη_a as a function of 1/T as shown in Figure 2(c) and the slope can be calculated using linear regression. ΔE values of CAB melt at the shear rate of 200, 500, 1000, and 2000 s⁻¹ were calculated to be 67, 42, 36, and 38 kJ/mol. CAB melt exhibited obvious higher melt flow activation energies at lower shear rates, indicating that the melt viscosity of CAB was more sensitive to temperature change at the range of lower shear rates.

The spinning temperature

It was found that the melt spinning of CAB could not be performed at 220℃, because the spinning extruder could not work due to a very high melt viscosity at such a low temperature. When spinning temperature was raised to 230 or 240℃, CAB fibers were successfully prepared. However, once the spinning temperature reached 250℃ or higher, a pungent odor was smelled during spinning and CAB melt turned to dark and yellow in color, implying the occurrence of thermal degradation of CAB during spinning.²⁵ Notably, as mentioned above, there was no obvious thermal degradation of CAB at 250℃ revealed by the thermogravimetric analysis. This demonstrated that thermal decomposition of CAB was facilitated by the shearing action and the relatively long residence time of CAB melt in the pipeline of spinning machine at high temperatures. These results suggested that the suitable temperature for melt spinning of CAB was between 230 to 240℃.

In order to further evaluate the thermal degradation of CAB during spinning, melt spinning of CAB was performed at different spinning temperatures (230 and 240℃) with a fixed melt extrusion rate. The photographs of the solution of CAB raw material and as-spun fibers in chloroform were shown in Figure 3(a). Apparently, the solution of raw material was clear and colorless. However, the solution of fibers, especially fibers prepared at 240℃, appeared to be turbid and showed a dark yellow color, indicating that thermal degradation of CAB occurred during spinning at 230 or 240℃. This was also confirmed by the gel permeation chromatography test results. As shown in Figure 3(b), the number-averaged molecular weight of the CAB raw material was 71910 g/mol, whereas that of as-spun fibers prepared at 230 and 240℃ was 62370 and 51700 g/mol, respectively, showing an obvious decline in molecular weight. These data showed that the thermal stability of CAB during spinning was quite sensitive to temperature change due to the lability of the polysaccharide backbone of CAB at high temperatures.^2,26 Therefore, in order to lower the level of thermal degradation of CAB and thereby maintain the good melt spinnability and fiber quality, it is best to conduct the melt spinning of CAB at a relatively low spinning temperature. All the CAB fibers reported below were prepared at 230℃.

Figure 3.

(a) Photographs of the solution of cellulose acetate butyrate (CAB) raw material and as-spun fibers prepared at 230 and 240℃ in chloroform (the concentration of all solution is 0.1g/ml). (b) the number-averaged molecular weight (M_n) of CAB raw material and as-spun fibers prepared at 230 and 240℃.

Orientation, crystallinity, and mechanical properties of the CAB as-spun fibers

The spinning draw ratio (SDR) is the speed of winding divided by the speed at which polymer melt is extruded out from spinneret. In this work, a series of CAB as-spun fibers with various SDR were prepared at 230℃ through adjusting melt extrusion rate and winding speed during spinning. As shown in Table 2, the linear density of as-spun fibers decreased from 929 to 53 dtex as SDR increased from 10 to 178. It was difficult to prepare fibers with a linear density less than 53 dtex under the current experimental condition. In order to prepare fibers with smaller linear density, a higher winding speed needs to be employed and this would lead to a higher stress in the spinline. However, according to tensile test results shown in Table 4, the highest tensile strength of as-spun fibers was as low as 1.18 cN/dtex. The stress in the spinline might exceed the breaking force of as-spun fibers with linear density less than 53 dtex, which could probably result in fiber breakage during spinning. Therefore, there was a limitation in linear density of CAB fiber prepared using a spinneret with a diameter of 1.0 mm in this work. The micrographs of the surface and cross-section of as-spun fibers with various linear density are shown in Figure 4. It can be seen that the melt spun CAB fibers had a very smooth surface and a solid interior structure.

Table 2.

The linear density and orientation results for cellulose acetate butyrate (CAB) as-spun fibers with various spinning draw ratios.

Spinning draw ratio	Linear density (dtex)	Sonic velocity (m/s)	Birefringence Δn × 10³
10	929	1657	0.82
31	305	1709	1.14
44	215	1896	1.17
104	90	1932	1.26
178	53	2120	1.42

Figure 4.

Scanning electron microscope (SEM) images of cellulose acetate butyrate (CAB) as-spun fibers with the linear density of (a, b) 305 dtex, (c) 215 dtex, (d) 90 dtex, and (e) 53 dtex.

The sonic velocity and birefringence (Δn) of as-spun fibers were measured to assess the overall degree of orientation of both amorphous and crystalline phases. The sonic velocity and birefringence data of as-spun fibers with different SDR are summarized in Table 2. Theoretically, since the increased SDR provides CAB melt thread on the spin-line with higher extensional strain rate, the orientation of the as-spun fibers ought to be improved with the increase of SDR. As shown in Table 2, the sonic velocity gradually increased from 1657 to 2120 m/s as SDR increased from 10 to 178. Meanwhile, Δn also increased with SDR. However, Δn of as-spun fiber with the highest SDR was even as small as 0.00142. This is reasonable as the birefringence is closely related to polymer segmental orientation. However, CAB segments are difficult to be associated with each other and oriented along flow direction during spinning, because the bulky acetyl and butyryl side groups are randomly placed along the polysaccharide chains which offers CAB macromolecules an extremely irregular steric structure.

The 2D WAXD patterns of as-spun fibers are shown in Figure 5. Two broad and rather fuzzy rings or arcs were observed in the X-ray patterns of all fiber samples, indicating that a certain quantity of crystallites was present in the fiber, although the haziness of the diffraction rings suggested that the degree of crystallinity was quite low. The very indistinct circles in the X-ray patterns reflected the poor crystallization ability of CAB due to the heterogeneity in the molecular segments along the polymer chains of CAB. Moreover, as shown in Figure 5, X-ray pattern of fiber with SDR = 0 (the SDR of as-spun fibers formed through drawing by its own gravity instead of the winding approximately equal to 0) exhibited complete rings. However, these closed rings, especially the inner rings, gradually evolved to arcs and became somewhat brighter at the left and right sides of the rings as SDR increased. This suggested that the crystalline phase became more oriented along fiber axis as SDR increased. However, no appreciable changes in width of the rings for all fiber samples can be observed. In other words, the bright areas of the rings did not tend to sharpen as SDR increased, indicating that the crystalline phases did not grow in size or became regular to any significant extent with the increase of SDR. Nevertheless, DSC detected the difference in crystallinity among those as-spun fibers with various SDR. As shown in Figure 6, only a glass transition was observed on DSC thermogram for fiber with SDR = 0. As SDR increased, the as-spun fibers displayed melting peaks. As listed in Table 3, the melting enthalpy for as-spun fibers with SDR = 31, 44, 104, and 178 was determined to be 0.5, 1.2, 1.4, and 1.8 J/g, respectively. This indicated that crystallinity of as-spun fibers indeed increased as the SDR increased. It has been reported that molecular orientation could induce crystallization of polymer fibers.^27,28 Therefore, the improved molecular orientation of CAB on the spin-line at higher winding speed (SDR) facilitated crystallization of as-spun fibers. Little has been reported in literature concerning the melt crystallization of CAB. Pizzoli et al.²⁹ and Sato et al.³⁰ found that CAB could crystallize from its blends with poly(3-hydroxybutyrate). Though CAB possessed very poor crystallization ability, this work proved that crystallization of pure CAB from melt was possible through improving molecular orientation of CAB.

Figure 5.

Two-dimensional wide-angle X-ray diffraction (2D WAXD) patterns of cellulose acetate butyrate (CAB) as-spun fibers with various spinning draw ratios.

Figure 6.

Differential scanning calorimetry (DSC) curves of cellulose acetate butyrate (CAB) as-spun fibers with various spinning draw ratios.

Table 3.

The melting point and melting enthalpy of cellulose acetate butyrate (CAB) fibers, employing data from differential scanning calorimetry (DSC) thermograms shown in Figures 6 and 10.

As-spun fibers			Drawn fibers
Spinning draw ratio	Melting point (℃)	Melting enthalpy (J/g)	Drawing temperature (℃)	Draw ratio	Melting point (℃)	Melting enthalpy (J/g)
0	–^a	–^a	Fibers before drawing		150^b	0.5^b
31	150	0.5	135	1.5	149	3.2
44	154	1.2	145	1.8	151	2.7
104	155	1.4	155	2.4	–^a	–^a
178	157	1.8

Note: ^a No discernable melting peak was observed on the DSC thermograms. ^b Data for as-spun fiber with a spinning draw ratio of 31.

Table 4.

Tensile test results of the cellulose acetate butyrate (CAB) as-spun fibers with various spinning draw ratios.

Spinning draw ratio	Tensile strength (cN/dtex)	Initial modulus (cN/dtex)	Breaking elongation (%)
10	0.80 ± 0.03	17.5 ± 0.6	29.0 ± 2.4
31	0.82 ± 0.02	19.6 ± 0.6	22.3 ± 2.2
44	0.93 ± 0.03	24.9 ± 1.0	15.2 ± 0.8
104	1.06 ± 0.03	26.5 ± 1.5	12.6 ± 1.1
178	1.18 ± 0.04	30.4 ± 1.6	8.2 ± 0.9

The mechanical performance of polymer fibers is closely related to their aggregation structure. From the above discussion, it was clear that the orientation and crystallinity of CAB as-spun fibers were improved as SDR increased. This implied that macromolecules of CAB were arranged more parallel along the fiber axis and packed more tightly and orderly for the as-spun fibers with higher SDR. Therefore, as shown in Table 4 and Figure 7, the tensile strength and initial modulus of as-spun fibers increased significantly with increasing SDR. It should be stressed that the highest tensile strength of as-spun fiber reached 1.18 cN/dtex. This value was much higher than those reported in literature.^22,31 Among commodity commercial fibers, CA fiber most resembles CAB fiber in terms of the fiber molecular structure. More importantly, tensile strength and initial modulus of the melt spun CAB fibers prepared in this work was comparable to the tensile strength (1.06∼1.23 cN/dtex)⁶ and initial modulus (22∼40 cN/dtex)⁵ of commercial CA fibers manufactured by costly dry spinning. Therefore, the melt-spun CAB fiber may be qualified to replace the solution-spun CA fiber in its traditional applications such as clothing and cigarette filters. Furthermore, since CAB fiber is biodegradable and non-toxic, it is potentially useful in biomedical material applications.

Figure 7.

Representative stress–strain curves of the cellulose acetate butyrate (CAB) as-spun fibers with various spinning draw ratios.

It was noteworthy that there was a trade-off between tensile strength and breaking elongation for as-spun fiber. For example, as SDR increased from 10 to 178, tensile strength of as-spun fibers increased from 0.78 to 1.18 cN/dtex, whereas breaking elongation markedly decreased from 29.0% to 8.2%. Since appropriate elongation of fibers is desirable in apparel applications, the CAB fiber with lower breaking elongation may not be suitable for clothing, although it showed relatively higher tensile strength. The degree of polymerization (DP) of the CAB raw material was as low as 225, though its number-averaged molecular weight reached 71910 g/mol. To prepare the CAB fibers with both higher tensile strength and sufficient breaking elongation, CAB with higher DP could be synthesized as the raw material for melt-spinning fibers.

Effects of hot drawing on the structure and properties of CAB fibers

As discussed above, the stretch of CAB melt thread during spinning exerted great impacts on the aggregation structure and mechanical properties of the as-spun fibers. The hot drawing process usually provides polymer fibers with a much higher degree of molecular orientation compared to the spinning process. Hence, significant changes in the structure and an improvement in mechanical performance are expected for CAB fibers after hot drawing.

In this work, CAB as-spun fibers with high SDR (e.g. SDR = 104 or 178) already had a relatively high degree of molecular orientation and thus were difficult to be hot drawn. Therefore, CAB as-spun fibers with relatively low SDR (SDR = 31) was chosen to undergo the hot drawing process. From DSC results as shown in Figure 6, it can be seen that the glass transition temperature of CAB was about 136℃, and the melting point of the as-spun fibers with SDR = 31 was about 150℃. Therefore, theoretically, the proper drawing temperature for the as-spun fibers with SDR = 31 was in the range of 136 ∼ 150℃. Actually, during the experiment, it was found that the CAB fiber was hard to stretch in hot glycerol when the temperature of the glycerol was lower than 135℃. This can be understood if one considers that segments of CAB could not move easily until the temperature exceeded its glass transition temperature (136℃). Meanwhile, as revealed by SEM micrographs (see Figure 8), the surface of the drawn fibers prepared at 145℃ was as smooth as that of the as-spun fibers, yet fibers drawn to a ratio of 1.5 at 155℃ showed a relatively rough and bumpy surface. This implied that the viscous flow of CAB happened at a temperature higher than its melting point (150℃) during drawing process. Therefore, the actual experiment results also suggested that the suitable drawing temperature was at 135∼150℃ for as-spun fiber with SDR = 31. Nevertheless, in order to study the influence of drawing temperature on the structure and mechanical performance of the drawn fibers, CAB fiber drawn at 155℃ was also prepared in this work.

Figure 8.

Scanning electron microscope images of (a) cellulose acetate butyrate (CAB) as-spun fibers and the corresponding drawn fibers prepared at (b) 145℃ with draw ratio of 1.8, and (c) 155℃ with draw ratio of 1.5.

The maximal draw ratio as well as the molecular orientation results of the drawn fibers prepared at 135, 145, and 155℃ are presented in Table 5. As shown in Table 5, though the fiber was only drawn to a ratio of 1.5 at 135℃, the sonic velocity of fibers was significantly increased from 1709 m/s to 2847 m/s. The birefringence also showed a large increase, indicating a significant improvement in fiber molecular orientation after drawing. The draw ratio reached 1.8 as the drawing temperature was raised to 145℃. Meanwhile, both the sonic velocity and birefringence of the drawn fiber prepared at 145℃ were higher than those of the as-spun fibers, but were lower than those of the drawn fiber prepared at 135℃. This may be attributed to the faster disorientation rate caused by shorter relaxation time of CAB molecules at higher temperatures. When the drawing temperature was set at 155℃, the sonic velocity and birefringence of corresponding drawn fibers were even lower than those of the as-spun fiber though the draw ratio reached 2.4. This may because the disorientation rate of CAB macromolecules is higher than their orientation rate during the drawing process when the drawing temperature exceeded the melting point of the as-spun fibers.

Table 5.

The maximal draw ratio and the orientation results of cellulose acetate butyrate (CAB) fibers drawn at different temperatures.

Drawing temperature (℃)	Draw ratio	Sonic velocity (m/s)	Birefringence Δn × 10³
–	–	1709^a	1.14^a
135	1.5	2847	1.71
145	1.8	2477	1.32
155	2.4	1556	0.69

Note: ^a Data for as-spun fiber with a spinning draw ratio of 31.

The effect of draw condition on the development of crystalline orientation was examined by 2D WAXD. As shown in Figure 9, the rings in X-ray patterns of CAB fibers showed an obvious tendency to focus at a particular spot by drawing at 135 and 145℃. Similar changes of X-ray patterns for stretched CA yarns were reported by Workt.³² However, X-ray patterns of the drawn fiber prepared at 155℃ very much resembled those of the as-spun fibers. The evolution of these X-ray patterns illustrated that orientation of crystalline areas occurred in CAB drawn fibers prepared under proper drawing temperature. The melting behavior of CAB drawn fibers is shown in Figure 10. The melting enthalpy was calculated to be increased from 0.5 J/g for as-spun fiber to 3.2 and 2.7 J/g for the drawn fibers prepared at 135 and 145℃, respectively (see Table 3). This suggested that the improved orientation of CAB fibers also induced further crystallization during drawing process. In addition, the crystallinity of the drawn fibers was found to be closely associated with their degree of molecular orientation. Crystallites in CAB as-spun fibers were supposed to be destructed by the rapid molecular relaxation during drawing under 155℃ and no discernable endothermic melting peak could be observed on the DSC curves.

Figure 9.

Two-dimensional wide-angle X-ray diffraction (2D WAXD) patterns of cellulose acetate butyrate (CAB) as-spun fiber and the drawn fibers prepared at different draw conditions.

Figure 10.

Differential scanning calorimetry (DSC) curves of cellulose acetate butyrate (CAB) drawn fibers prepared at different draw conditions.

Tensile tests of CAB drawn fibers were performed to understand the effect of structural change on mechanical properties. Tensile test results are given in Table 6 and typical stress–strain curves are shown in Figure 11. Responding to the improved molecular orientation and crystallinity, drawn fibers prepared at 135℃ and 145℃ showed a substantial improvement in tensile strength and initial modulus compared to the undrawn fibers. Meanwhile, breaking elongation of the fibers drastically declined after drawing. For example, the tensile strength of CAB fibers was significantly increased from 0.82 cN/dtex for the undrawn fiber to 1.42 cN/dtex for the drawn fiber prepared at 135℃, whereas the breaking elongation was decreased from 22.3% to 5.0%. As a result of reduction in the degree of molecular orientation and crystallinity, CAB drawn fiber prepared at 155℃ even showed a lower tensile strength (0.70 cN/dtex) compared to the undrawn fiber.

Table 6.

Tensile test results of cellulose acetate butyrate (CAB) drawn fibers prepared at different draw conditions.

Drawing temperature (℃)	Tensile strength (cN/dtex)	Initial modulus (cN/dtex)	Breaking elongation (%)
–	0.82 ± 0.02^a	19.6 ± 0.6^a	22.3 ± 2.2^a
135	1.42 ± 0.04	40.7 ± 1.8	5.0 ± 0.7
145	1.15 ± 0.03	29.4 ± 2.8	10.7 ± 1.4
155	1.22 ± 0.04	32.7 ± 2.2	8.2 ± 1.1
–	0.70 ± 0.09	20.1 ± 3.4	12.1 ± 4.4

Note: ^aData for the as-spun fibers with a spinning draw ratio of 31.

Figure 11.

Representative stress–strain curves of cellulose acetate butyrate (CAB) drawn fibers prepared at different draw conditions.

Dyeability of CAB fibers

Disperse dyes are almost water-insoluble dyes with substantivity for cellulose ester fibers.^33,34 In this study, the Dianix Blue ACE, a kind of disperse dyestuff, was used to evaluate the dyeability of CAB fiber. It is well known that dyeing temperatures usually has a profound impact on dyeability of polymer fibers. The dye uptake of the Dianix Blue ACE dye on CAB as-spun fiber (SDR = 44) at various dyeing temperatures (60 ∼ 100℃) is shown in Figure 12(a). It was clear that the dye uptake at 60 and 70℃ was around 20%. As the dyeing temperature increased from 70 to 90℃, the dye uptake increased obviously from 22% to 48.2%. It is generally considered that a dye molecule diffuses in the amorphous region of a fiber through holes produced as the result of segmental motions.³⁵ Therefore, the increased dye uptake with the increase of dyeing temperature was mainly attributed to the improved mobility of CAB segments. The difference in the uptake of the disperse dyes at various dyeing temperatures was directly reflected by photographs of the dyed CAB fibers. As shown in Figure 12(b), the fibers dyed at 60 and 70℃ appeared to be light blue. The blue color of dyed fibers was gradually deepened with the increase of dyeing temperature from 70 to 90℃. It can be seen that the dye uptake was only slightly improved when the dyeing temperature was increased from 90 to 100℃. Considering that the delustering and thermal shrinkage of cellulose ester fiber may occur at high temperatures,³⁶ the optimum dyeing temperature for CAB fiber was between 80 and 90℃.

Figure 12.

(a) The dye uptake and (b) photographs of cellulose acetate butyrate (CAB) as-spun fibers dyed at 60, 70, 80, 90, and 100℃.

Conclusions

The thermostability and rheological properties of CAB were studied before the successful preparation of CAB fiber without any additives through melt spinning. Oxygen in air could facilitate thermal degradation of CAB. CAB melt was difficult to flow until the temperature reached 230℃. CAB was a fluid with strong shear thinning behavior and with a viscosity more sensitive to temperature at relatively low shear rates. The declined molecular weight of CAB revealed the occurrence of thermal degradation during melt spinning, though thermogravimetric analysis showed that there was no obvious weight loss at the temperature range for spinning (230∼250℃). The thermostability of CAB during spinning was found to be very sensitive to the spinning temperature. To suppress the decomposition of CAB, melt spinning of CAB should be conducted at the relatively low temperature of 230℃. Molecular orientation of CAB fibers was improved through increasing the spinning draw ratio during spinning process, or through a hot drawing process. Meanwhile, the crystallinity of CAB fibers was found to be increased as the orientation was improved. Tensile strength and initial modulus of the fibers were enhanced due to the improved orientation and crystallinity. CAB fiber could be dyed with a disperse dye, and the optimum dyeing temperature was determined to be between 80∼90℃.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors gratefully acknowledge the valuable help and great support from National Natural Science Foundation of China (No.21404024), China Postdoctoral Science Foundation (No.2016M591573) and the Fundamental Research Funds for the Central Universities (No.2232015G1-11 and No.2232014D3-20).

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