Impact of hot-stretching treatment on physical and mechanical properties of braided polyamide suture

Abstract

Suture is the most used implant in the human body, but it is not yet perfect. Braided polyamide sutures have to undergo several manufacturing steps, including hot-stretching. The main objective of this work was to analyze physical structural changes of polyamide fibers after the hot-stretching step and to highlight mechanical property changes related to this treatment. The obtained results showed that hot-stretching of polyamide braided suture allows one to obtain uniform and compact suture. In this paper, modification of mechanical properties due to hot-stretching has been correlated to macromolecular properties, such as crystallinity, crystal size and macromolecular chain orientation. Hot-stretching braided suture at a temperature higher than 200℃ and the medium value of drawing ratio resulted in the best braided suture properties. Below 180℃, polyamide braided suture undergoes thermal shrinkage during hot-stretching, leading to an increase in breaking extension. Hot-stretching suture ameliorated fabricated braided suture crystallinity reached 33%, with the creation of numerous crystals with smaller size. Using an overlaid contours plot, we have determined optimal values for significant manufacturing parameters in order to manufacture polyamide braided suture that has high ultimate tensile strength, low extension at break, low rigidity and high crystallinity.

Keywords

polyamide braided suture hot-stretching mechanical properties differential scanning calorimetry melting peak characteristics

Suture material has been used for at least 4000 years.¹ Archaeological records from ancient Egypt show that Egyptians used linen and animal sinew to close wounds.^1,2 The classical suture materials are protein catgut and silk. However, with the advent of new polymers over the past 50 years, new sutures have been developed and introduced periodically. These synthetic sutures are generally semi-crystalline polymers and the morphology associated with these materials results in the desired mechanical properties, such as stiffness and toughness.³ The first-used polymeric suture is polyamide suture, which is a non-absorbable suture, commonly used in dermatologic surgery.⁴

The pliability characteristics of these sutures permit good handling. Because nylon sutures are more pliable and easier to handle than polypropylene and polyester (PET) sutures, they are favored for the construction of interrupted percutaneous suture closures. Nylon sutures are made from both monofilament and braided structures. Monofilament sutures have a smooth surface, from which knots can be easily undone, and they suffer from relatively high stiffness, which creates problems for surgeons during knotting. Braided sutures have a flexible structure. However, they have a rough surface that relieves higher tissue reactivity. Moreover, they have a greater tendency to break, in spite of being more flexible than monofilament suture.^5,6

In order to improve their surface characteristics, braided polyamide 6-6 sutures are generally compacted by stretching steps.⁷ In this treatment, suture is subjected to a stretching force causing a decrease of suture diameter by making it more compact.^7,8 The stretched braided sutures offer many advantages over non stretched sutures, notably in their flexibility and security.⁸ Hot-stretching suture makes it possible to prepare the strongest suture for a given suture diameter by compacting the braided suture and thus reducing the volume of the voids therein. Another advantage of this treatment is that it reduces capillarity and allows one to obtain a smooth surface involving generally less tissue trauma.^7–10

Literature concerning hot-stretching braid suture is extremely rare. Much research has focused on determining the effect of drawing and heat treatment conditions on the properties of fibers or yarns,^11–15 but few studies are currently available regarding the braid suture hot-stretching. Physical parameters and materials used during industrial hot-stretching processes are unknown and generally kept secret by the few suture manufacturers. The effectiveness of any method of stretching is limited by the percentage of stretching that can be achieved without substantially damaging the threads of the suture. In fact, the greater the percentage stretch without damaging the threads, the greater the decrease in diameter of the suture and the greater the increase in tensile strength and knot strength.⁷ Synthetic suture is stretched under high temperature to allow easy polymeric chain deformation.^7,9 Hain et al.¹⁶ proposed to subject suture strands to dynamic treatment by passing the suture with multiple turns around at least two godets located within a heating zone provided by an oven. Washington and Entrekinr⁹ developed a new technique for ligature hot-stretching. They submitted PET ligatures to progressive stretch and successive steps of heat treatment. The developed device is composed of a series of rolls with incremented temperature and speed. In these earlier researches, the effect of hot-stretching treatment conditions on braid tensile strength was investigated. Dependence was confirmed between the temperature and draw ratio and tensile strength mechanical properties. However, the authors have not studied the impact of this treatment on thermal and all mechanical tensile properties of braided suture.

In the classical process, multifilament yarn drawing is performed before the braiding step. So, much research has focused on polyamide filament heat treatment and studied the effect of filament drawing during the extrusion step on suture mechanical properties.¹⁷ Few researches investigated the effect of hot-stretching treatment applied to the braided structure on braided suture properties. The effect of thermal treatments on physical, chemical and morphological properties of braided suture has been rarely treated in the literature. In our previous research, we demonstrated that the hot-stretching treatment resulted in significant changes in the performance of polyamide braided suture.⁷ Furthermore, we found that this treatment affects mechanical properties and we demonstrated that the failure of these materials may be in part due to alterations of the polymer and the braided structure caused by these treatments. In this study, drawing with simultaneous heat treatment of the braid yarns was performed to braid the structure in order to achieve the desired properties . The main aim of this research was a detailed study of the effect of hot-stretching step conditions on the mechanical properties of polyamide braided suture. The influence of the hot-stretching step on physical properties of suture polymeric materiel, such as crystallinity and thermal transitions, was also investigated. Optimal conditions were determined by the simultaneous optimization method of mechanical and physical properties of polyamide braided suture.

Materials and methods

Braided suture manufacturing

Braided sutures made of 16 non-texturized polyamide 6-6 yarns 44 dtex f 23 were fabricated using a HERZOG circular braiding machine with 16-carrier arrangement and a cogwheel ratio of 0.064. The cogwheel ratio adjustment corresponds to the adjustment of the braid driving speed. We implemented a 1/2 interlacing braid geometry (Figure 1(a)) obtained through 1/1 sequential carrier motion. Braids were then cleaned by a scouring treatment and compacted by hot-stretching.

Figure 1.

Digital captured images enlarged 40×: (a) non hot-stretched braided suture; (b) hot-stretched braided suture.

In the hot-stretching step, we have used a heat setting machine developed a by local textile braided cables manufacturer (Figure 2). Braided suture is supported by a braid roll (1) and passes around feed rollers (2). Then it is drawn around the take-up rollers (5) and collected on a take-up spool (6). Braided suture is subjected to the thermal source (4), inside the thermal treating chamber (3). The braided suture internal drawing ratio is accomplished by rotating the take-up rollers (5) faster than the feed rollers (2) and is calculated by Equation (1). The heat setting is accomplished by the action of high temperature in the thermal chamber and quenching between the take-up rollers and take-up spool. The drawing intensity depends on braided suture size, suture material and braid construction. Similarly, the temperature and residence time depend also on these same factors:

Internaldrawingratio (%) = \frac{(Take_up_rollers_rate - Feed_rollers_rate) \times 100}{Take_up_rollers_rate}

(1)

Figure 2.

Hot-stretching machine.

For braided suture hot-stretching, Washington and Entrekinr⁹ suggested a temperature between 10℃ and 65.5℃ below the melting point of the braided suture material. Vasanthan¹⁵ reported that a sharp increase in the crystallinity occurs at a temperature above 160℃ for both polyamide 6-6 and polyamide 6 fibers, suggesting that thermal treatment occurs at temperatures above 160℃. We have used a hot-stretching temperature above 160℃ and below melting temperature.

According to Cook¹⁸ braided and twisted structures can be stretched under a drawing ratio between 15% and 35%. Washington and Entrekinr⁹ showed that the drawing ratio of braided suture should be between 6% and 25%. Thompson⁸ reported that silk braided suture can be stretched up to 2.5 times. In our study, we used a drawing ratio between 10% and 90% in order to study the effect of the internal drawing ratio.

Residence time in the treatment chamber and take-up spool rate were also changed in order to study the effect of heat and quenching time on the physical and the mechanical braided suture properties. In this study, we used a response surface design method with a central composite design plan. Five levels for every factor were used (Table 1). An experimental conditions plan is presented in Table 2. Hot-stretching at 220℃ and 240℃ was performed in order to optimize hot-stretching conditions, as will be described in the Results and discussion section.²

Table 1.

Levels of hot-stretching conditions according to a central composite design plan

Factor	–2	–1	0	1	2
Temperature (℃)	160	170	180	190	200
Internal drawing ratio (%)	10	30	50	70	90
Residence time (min)	0.63	0.75	0.94	1.25	1.88
Take-up spool rate (m min^–1)	7	9.5	10.5	11	11.75

Table 2.

Experimental conditions

	Temperature (℃)	Take-up spool rate (m min^–1)	Residence time (min)	Internal drawing ratio (%)
PA-1	160	10.5	0.94	50
PA-2	170	9.5	0.75	30
PA-3	170	9.5	0.75	70
PA-4	170	9.5	1.25	30
PA-5	170	9.5	1.25	70
PA-6	170	11	0.75	30
PA-7	170	11	0.75	70
PA-8	170	11	1.25	30
PA-9	170	11	1.25	70
PA-10	180	10.5	0.94	90
PA-11	180	10.5	0.94	50
PA-12	180	10.5	0.94	10
PA-13	180	10.5	0.63	50
PA-14	180	10.5	1.88	50
PA-15	180	7	0.94	50
PA-16	180	11.75	0.94	50
PA-17	190	9.5	0.75	30
PA-18	190	9.5	0.75	70
PA-19	190	9.5	1.25	30
PA-20	190	9.5	1.25	70
PA-21	190	11	0.75	30
PA-22	190	11	0.75	70
PA-23	190	11	1.25	30
PA-24	190	11	1.25	70
PA-25	200	10.5	0.94	50
PA-26	220	10.5	0.94	50
PA-27	240	10.5	0.94	50

Suture diameter was measured according to the USP32-NF27 S2 <861> method described in the USP32-NF27 S2 monograph for non-absorbable sutures. A dead weight mechanical thickness gauge (SODEMAT-Troyes-France), equipped with a direct digital reading and graduated in 0.02 mm divisions, was used. The developed hot-stretched braided sutures (Figure 1(b)) have a diameter between 0.20 and 0.299 mm. According to the USP32-NF27 S2 monograph, fabricated braided sutures have a USP number of 3-0.²

Determination of mechanical properties

We used a tensile testing machine (Dynamometer LLOYD, England) with a constant rate of extension. All tensile tests were performed according to ASTM D76-99 standard test methods. The load cell was chosen such that the tensile force of tested braided sutures was between 10% and 90% of the load cell’s capacity.¹⁹ All tests were carried out in a controlled environment of 21 ± 1℃ temperature and 65% ± 2% relative humidity per ASTM D 1776-04.²⁰ We used a straight pull tests procedure adopted from Instron’s test method and explained in previous works.^21,22 The braided suture specimen undergoes a longitudinal traction until rupture at a strain rate of 300 mm/min²¹ with a 100 N load cell and 150 mm initial gauge length. We tested 10 specimens and determined the average value and the standard deviation of breaking load (N), ultimate tensile strength (UTS) (MPa), Young's modulus (GPa) and extension at break (%). These proprieties are defined as follows:

– breaking load (N) is defined by the force that leads to braided suture breaking;

– UTS (MPa) is determined by the ratio between the breaking load and braided suture section;

– extension at break (%) is the measured extension when the break occurs;

– the Young’s modulus is the elastic modulus determined by the ratio of the stress (force per unit area) over the strain (ratio of deformation over initial length) in the first stress–strain curve zone.

Determination of physical properties using differential scanning calorimetry

In order to study the impact of hot-stretching of braided polyamide suture on polymer physical properties, such as crystallinity and melting peak properties, we have used differential scanning calorimetry (DSC: Mettler Toledo 832^e). Samples were cut in small stumps (less than 10 mg) and were packed in perforated aluminum pans to ensure good heat diffusion. For all tested samples, we have considered the following sequence. Firstly, the temperature was increased from 25℃ to 300℃ with a heating rate of 10℃.min^–1. Then the temperature was reduced to 25℃ with a cooling rate of 10℃.min^–1. Heating curves allow one to determine melting peak characteristics, such as heat flow integral (J/g), crystallinity (%), melting peak temperature (℃), onset temperature (℃), melting peak width (℃) and melting peak height (W.g^–1). Figure 3 illustrates melting peak characteristics defined as follows.^23–25

– Melting peak temperature (℃): the minimum point of the melting peak.

– Onset peak temperature (℃): obtained by the intersection between the extrapolated initial baseline and curve tangent. It presents the temperatures of melting beginning.

– Peak height (W.g^–1): measured between interpolated baselines and the melting peak. It gives an idea about the degree of polymer orientation.

– Peak width (℃): measured in the medium peak high. It gives an idea about the average polymer crystal size.

– Melting heat flow integral (J.g^–1): the melting enthalpy determined by the integral of melting peak divided by sample mass.

– Crystallinity (%): obtained by the ratio of melting enthalpy and the enthalpy of the ideal crystal (ΔHf^∞= 196 J.g^–1).

Figure 3.

Terms describing melting peak characteristics illustrated by an example of a differential scanning calorimetry curve of polyamide 6-6.

Statistical analysis applied to response surface experimental designs permitted one to identify most important factors for each mechanical and physical property of braided polyamide suture. Statistical analysis has been realized by using Minitab16 software (Minitab Ltd, UK).

The R-squared values (R²) are equal to 0.71, 0.56, 0.66 and 0.51, corresponding respectively to the extension at break, UTS, Young’s modulus and crystallinity responses. These values show a high level of correlation between the model’s responses and the manufacturing parameters. Identification of the significant effects was performed through the Student test. It showed that there are some P-values that are higher than the significance level (5%), and they are eliminated.

Results and discussion

The primary purpose of the present paper is to study the effect of the hot-stretching step on mechanical and physical properties of braided polyamide sutures. Tables 3 and 4 illustrate obtained results related to the stress–strain test and DSC analysis. Most important factors for each response were identified from main effects plots generated by factorial analysis, presented in Figures 4 and 5.

Table 3.

Mechanical properties of hot-stretched braided sutures

Fabricated sutures	Diameter (mm)	Breaking load (N)	UTS (MPa)	Extension at break (%)	Young’s modulus (GPa)
PA-non hot stretched	0.26 ± 0.005	24.70 ± 0.45	465.46 ± 5.45	44.73 ± 1.4	2.02 ± 0.10
PA-1	0.285 ± 0.004	28.91 ± 0.63	453.39 ± 4.63	25.49 ± 2.14	2.31 ± 0.29
PA-2	0.275 ± 0.005	31.60 ± 0.34	532.37 ± 5.34	26.60 ± 1.09	2.64 ± 0.16
PA-3	0.270 ± 0.005	33.30 ± 0.35	581.82 ± 5.35	21.03 ± 1.16	3.01 ± 0.17
PA-4	0.255 ± 0.003	30.75 ± 0.28	602.43 ± 3.28	28.80 ± 1.11	3.29 ± 0.19
PA-5	0.265 ± 0.015	33.22 ± 0.21	602.70 ± 15.21	20.32 ± 0.31	3.20 ± 0.08
PA-6	0.265 ± 0.004	31.40 ± 0.48	569.51 ± 4.48	34,01 ± 1.78	2.88 ± 0.16
PA-7	0.265 ± 0.005	35.75 ± 0.13	648.44 ± 5.13	16.02 ± 0.22	3.59 ± 0.05
PA-8	0.240 ± 0.010	31.47 ± 0.37	696.09 ± 10.37	27.91 ± 1.08	3.83 ± 0.23
PA-9	0.260 ± 0.010	29.97 ± 0.27	564.70 ± 10.27	20.98 ± 0.63	1.94 ± 0.09
PA-10	0.235 ± 0.005	31.08 ± 0.57	716.88 ± 5.57	17.09 ± 0.60	4.13 ± 0.23
PA-11	0.280 ± 0.011	30.38 ± 0.49	493.60 ± 11.49	35.52 ± 0.48	2.30 ± 0.06
PA-12	0.280 ± 0.010	30.18 ± 0.25	490.33 ± 10.25	42.39 ± 1.35	1.95 ± 0.09
PA-13	0.298 ± 0.010	31.12 ± 0.05	440.54 ± 10.05	36.25 ± 0.31	2.22 ± 0.03
PA-14	0.265 ± 0.005	31.20 ± 0.31	566.03 ± 5.31	25.12 ± 1.91	3.28 ± 0.38
PA-15	0.290 ± 0.010	31.24 ± 0.48	473.21 ± 10.48	33.47 ± 0.12	2.51 ± 0.04
PA-16	0.275 ± 0.003	30.90 ± 0.23	520.51 ± 3.23	31.24 ± 1.06	2.85 ± 0.15
PA-17	0.285 ± 0.004	30.00 ± 0.42	470.53 ± 4.92	35.69 ± 1.47	2.13 ± 0.14
PA-18	0.285 ± 0.005	30.17 ± 0.35	473.18 ± 5.35	33.26 ± 1.05	2.02 ± 0.10
PA-19	0.290 ± 0.010	29.82 ± 0.61	451.71 ± 10.61	36.56 ± 2.50	1.79 ± 0.19
PA-20	0.298 ± 0.010	28.59 ± 0.46	404.67 ± 10.46	27.37 ± 0.77	1.83 ± 0.08
PA-21	0.265 ± 0.005	30.06 ± 0.68	545.29 ± 5.68	34.55 ± 1.88	2.59 ± 0.22
PA-22	0.260 ± 0.010	31.08 ± 0.35	585.72 ± 10.35	22.96 ± 1.23	3.43 ± 0.28
PA-23	0.280 ± 0.010	30.13 ± 0.44	489.56 ± 10.44	35.78 ± 1.78	2.27 ± 0.17
PA-24	0.265 ± 0.003	29.77 ± 0.97	540.08 ± 4.47	21.32 ± 2.66	2.72 ± 0.52
PA-25	0.235 ± 0.004	33.57 ± 0.21	774.40 ± 4.71	16.85 ± 0.57	3.69 ± 0.19
PA-26	0.210 ± 0.010	32.32 ± 0.51	933.58 ± 10.51	15.60 ± 0.30	4.74 ± 0.16
PA-27	0.215 ± 0.015	34.63 ± 0.27	954.46 ± 15.27	11.65 ± 0.30	5.03 ± 0.20

UTS: ultimate tensile strength.

Table 4.

Melting peak characteristics of hot-stretched braided suture

		Heat flow of melting peak (J.g^–1)	Crystallinity (%)	Melting peak temperature (℃)	Onset melting peak temperature (℃)	Melting peak height (W.g^–1)	Melting peak width (℃)
PA-non hot stretched	Pic 1	32.281	16.470	249.250	246.120	1.210	3.750
PA-non hot stretched	Pic 2	4.136	2.110	255.970	254.020	0.270	2.480
PA-1		49.843	25.430	251.040	239.830	0.840	8.610
PA-2		71.520	36.490	249.130	242.000	1.230	8.510
PA-3		39.004	19.900	246.450	242.000	0.670	7.330
PA-4		62.700	31.990	249.460	240.440	1.180	7.750
PA-5		43.000	20.610	252.010	237.950	0.950	4.970
PA-6		68.032	34.710	248.640	240.690	1.480	6.770
PA-7		46.040	23.490	250.300	236.760	0.690	12.790
PA-8		53.802	27.450	250.810	240.420	0.890	9.900
PA-9		43.845	22.370	250.000	243.910	0.900	7.740
PA-10		73.559	37.530	249.620	240.330	1.080	10.800
PA-11		56.448	28.800	249.760	241.760	1.090	7.640
PA-12		42.748	21.810	249.590	243.070	1.020	6.130
PA-13		60.015	30.620	248.820	241.760	1.130	7.670
PA-14		72.481	36.980	249.450	239.890	1.180	8.650
PA-15		48.784	24.890	250.810	241.480	0.880	8.520
PA-16		65.974	33.660	249.300	239.460	1.020	9.210
PA-17		67.777	34.580	249.270	238.910	1.230	7.540
PA-18		45.492	23.210	250.950	243.240	1.010	6.650
PA-19		42.101	21.480	251.460	242.210	0.860	7.580
PA-20		69.972	35.700	249.470	240.930	1.310	7.210
PA-21		42.826	21.850	249.620	244.420	0.990	6.590
PA-22		69.952	35.690	250.160	238.440	1.200	8.260
PA-23		57.193	29.180	249.100	239.790	1.210	6.760
PA-24		60.309	36.000	249.790	241.050	1.170	7.350
PA-25	Pic:1	84.320	13.30	249.940	239.730	0.74	5.780
	Pic:2	117.44	19.93	253.930	250.230	0.56	3.230
PA-26	Pic:1	45.531	25.230	244.850	238.120	1.160	6.120
PA-26	Pic:2	11.094	5.660	252.880	250.160	0.630	2.780
PA-27	Pic:1	30.694	15.660	243.470	237.630	0.820	5.990
PA-27	Pic:2	12.309	6.280	252.310	249.380	0.550	3.520

Figure 4.

Mean effects plots for braided suture mechanical properties: (a) breaking load (N); (b) ultimate tensile strength (MPa); (c) Young's modulus (GPa); and (d) extension at break (%).

Figure 5.

Mean effects plots for melting peak characteristics: (a) heat flow integral (Jg^–1); (b) crystallinity (%); (c) onset melting peak temperature (℃); (d) melting peak temperature (℃); (e) melting peak height (Wg^–1); (f) melting peak width (℃).

Figure 6.

Contour plots of (a) extension at break (%), (b) ultimate tensile strength (MPa), (c) Young's modulus (Nm^–1) and (d) crystallinity (%).

Figure 7.

Overlaid contours plots of Young's modulus (GPa), ultimate tensile strength (MPa), extension at break (%) and crystallinity (%).

Figure 4(a) confirms that braided suture breaking load is ameliorated after the hot-stretching step in all conditions. It can also be seen that temperature is the most important factors and breaking load is ameliorated by increasing temperature. Mechanical properties of polymer materials are known to depend powerfully on the orientation and extension of their molecules chain.¹⁵ Consequently, the extension and orientation of chains during the hot-stretching step gives rise to an increase of braided suture strength. Frank and Wendorff²⁶ reported that after polymer hot-stretching, mechanical properties become predominantly determined by those of the strong covalent bonds along the chain backbone rather than by the weak Van der Waals bonds between neighboring chain molecules. Similarly, the UTS is increased under higher temperature (Figure 4(b)). The highest UTS value is equal to 774.40 ± 4.71 MPa, obtained by hot-stretching braided suture at a temperature of 200℃, is 66% higher than that of the non-hot-stretched sample. We note also the amelioration of UTS (716.88 ± 5.57 MPa) by the use of a high internal drawing ratio (90%), involving simultaneous decrease of the braided suture diameter (0.235 mm) (Table 3) and increase of breaking load (31.08 N). It can be seen that both drawing and thermal treatment lead to an improvement of the mechanical properties, which is in agreement with numerous reports.^{7,9,15,27–33}

When subjected to high temperature, polyamide 6-6 braid exhibits some tendency to shrink.^12,15,34 This explains why breaking extension increased in the case of braided sutures hot-stretched under temperatures below 180℃ (Figure 4(d)). This is due to the fact that polyamide 6-6 is a semi-crystalline polymer and partially oriented in the fiber axis direction. This anisotropic state of fiber is the state of low entropy. However, when braided sutures are subjected to thermal treatment, a tendency to go toward the state of higher entropy is observed.³¹ In amorphous regions, the state of molecular orientation is not stable and this is the reason for fiber shrinkage due to thermal disturbance during heat treatment. Various researchers have shown that braid extension at break for free-annealed samples depends on crystallinity, as well as on molecular orientation. It has been also reported that thermal shrinkage behavior of polymeric yarns at relatively low temperatures is dominated by the tendency of straight parts of molecules present in the mobile amorphous phase to coil in order to gain entropy.¹⁵

In the case of hot-stretching at a temperature higher than 180℃, relaxation of locked-in stress generated during manufacturing is accentuated and this leads to a more stable structure and hence less residual shrinkage. This change in crystal structure due to heat treatment can be the cause of decrease in breaking extension under temperatures higher than 180℃ (Figure 4(d)). Rath et al.,³¹ who proved that shrinkage decreases with heat treatment under high temperature at fixed load for PET tire cord, reported the same phenomenon. Vasanthan¹⁵ demonstrated also that at a high temperature less shrinkage occurs for samples heat-treated at fixed length than for those heat-treated under free shrinkage conditions, because of the presence of a greater degree of residual stress in these fibers. We note also that braided suture is easily extensible under high time, so braided suture is easily stretched under temperatures higher than 180℃ and shrinkage during heat treatments is reduced.

Khlif et al.³⁵ reported that polymer yarns shrinkage depends only on the amorphous volume fraction and amorphous orientation factor. So the decrease of breaking extension when increasing drawing ratio and residence time is generated by a decrease of amorphous volume fraction. At room temperature the thermal vibrations are sufficient to cause spontaneous rupture of hydrogen bonds and an orientation of macromolecular chains occurs under applied stress.³⁶ During hot-stretching, multifilament yarns are fixed in oriented shape and consequently breaking extension will be reduced. We note also that the increase of hot-stretching time induces higher structure fixation and lower structure extensibility.

Young's modulus of hot-stretched samples is 50% higher than that of non-hot-stretched braided suture (Figure 4(c)). In the case of hot-stretched braided suture at a temperature of 200℃, the Young's modulus attends a value of 3.69 ± 0.9 GPa. The observed considerable improvement of mechanical properties of hot-stretched braided suture is related to chain orientation. The same improvement of the mechanical properties was observed by other researchers, in the case of PET films³⁷ and polyamide 6 filaments.³⁸

Factorial analysis shows an increase of braided suture rigidity (Figure 4(c)), illustrated for example by an increase of Young’s modulus from 2.02 to 4.13 GPa after hot-stretching of the braided suture under an internal drawing ratio of 90% (Table 3). The Young's modulus increases as the internal drawing ratio increases at high temperatures. The results are in good agreement with those of Dumbleton and Murayama,²⁷ who studied drawing of nylon 6-6 using a Vibron direct reading viscoelastometer.

During manufacturing, polymeric fibers are formed into molecular structures having highly crystalline and oriented chains in the fiber direction. Although polymeric fibers are highly oriented and crystallized, amorphous regions still exist in the fibers, and these regions significantly influence the physical properties of the braided sutures during the hot-stretching step. Molecular structures of braided suture subjected to the hot-stretching step are analyzed by DSC. Melting peak characteristics of all hot-stretched braided sutures are presented in Table 4. According to Figure 5(b), it is clear that the temperature had a significant effect on each response. Crystallinity degree is also significantly influenced by temperature and by the internal drawing ratio during the hot-stretching step. This can be explained in terms of change in chain folding and crystal imperfection present in the braid with the change in heat treatment conditions. We note also that the degree of polyamide braided suture crystallinity increases rather strongly from about 18.58% to 37.53% because of the applied stress. The development of polymer crystallites after thermal treatments was also observed by several researchers^{15,26,31,35,39} who noticed that at high heat setting temperatures, the molecular mobility is high and crystal grows under the action of the alignment and closeness of certain segments within macromolecular chains to each other. The crystallites growth depends mainly upon the thermal energy brought to the macromolecules. Khlif et al.³⁵ demonstrated that PET fibers heat set at low temperature show low crystallinity build up of many small crystals. Yarn heat set at a high temperature creates few big crystals and large adjacent amorphous domains and relatively high overall crystallinity. Dumbleton and Murayama²⁷ proved also that the crystallinity increases with an increase in annealing temperature. Our work confirmed the significant positive effect of temperature on the crystalline arrangement of treated specimens. This phenomenon is accentuated by prolonging residence time to 1.88 min (Figure 5(b)). This can be explained by an increase of crystallite number. Gupta and Kumar²⁸ showed that the crystallite size increases with initial heat setting time, and then in most cases decreases with further time prolongation. Consequently, further prolongation of hot-stretching time can cause an increase of the number of crystals having small size.

Results in Figure 5(b) prove that high take-up spool (rapid cooling) involves a better crystallinity. These results were also reported by Fakirov,¹¹ who proved also that rapid fiber cooling after extrusion involves better drawability, and consequently better crystallinity.

Figure 5(d) shows factorial effects on melting peak temperature. The shown variation of meting peak temperature is due to the existence of crystals with different sizes.²⁵ From this figure we can report that an increase of take-up spool rate decreased melting peak temperature. This can be explained by the formation of small imperfect crystals by rapid quenching (faster take-up spool rate; Figure 5(d)). This statement was also demonstrated by Morton and Hearle,³⁶ who showed that a small crystal is formed by rapid quenching in the case of synthetic fiber. They reported also that the melting-point is lower with small imperfect crystals.³⁶

The exposure of braided suture to high heat temperature, near melting point, for longer time, involves the melting of the smallest and least perfect crystals and allows the growth of larger and more perfect ones. This allows a molecular rearrangement, a removal of defects and leads to bigger and better crystalline regions. This explains the fact that melting peak temperature increases by the increase of residence time, as shown in Figure 5(d).

Figure 5(d) illustrates an increase of melting peak temperature with the augmentation of hot-stretching temperature at temperature up to 160℃, proving the valley of crystal size. This phenomenon can be explained as follows: when the temperature is higher, the mobility of the chain becomes greater and more crystals having small sizes are formed in the amorphous region. So, the average crystallite size decreases and consequently melting peak temperature will decrease.^40,41 It happens also that onset melting peak temperature can increase slowly with the increase of hot-stretching temperature (Figure 5(c)). This proves the growth of size of the smallest crystals that melt first during DSC analysis.

Reported results from Figures 5(e) and (f) show that the increase of hot-stretching temperature involves an augmentation of the melting peak height and a decrease of peak width. This can be explained by the formation of oriented crystals with uniform size. Thus, thin peaks prove the existence of crystals with uniform size that melt at the same temperature. The high peak is explained by the increase of the amount of uniform crystals. From Figure 5(f), we deduce that the increase of the internal drawing ratio involves an increase of melting peak width as a consequence of formation of small crystals having irregular size. This can be explained by the fact that applied stress to the tie-segments, which link the crystalline regions, will help to break up the smaller crystals.³⁶

The contours plots performed from models of most important properties are presented in Figure 6. The phenomenon described in the previous paragraph can also be deduced from contour plot curves. For example, we can see that extension at break decreases with increasing internal drawing ratio (Figure 6(a)). The highest UTS is obtained by subjecting braided suture to a high internal drawing ratio, which reduces the braided suture diameter and increases breaking load (Figure 6(b)). However, the increase of the internal drawing ratio involves a high Young's modulus accentuated by an augmentation of residence time, as shown in Figure 6(c). Braided suture becomes stiffer by increasing the drawing ratio and residence time.

From the crystallinity contours plot (Figure 6(d)) we deduce that the highest values of crystallinity are obtained by using the highest values of internal drawing ratios and temperatures. We can deduce that both temperature and drawing ratio during hot-stretching lead to an improvement of the mechanical properties by elimination of defects in the structures. This is in agreement with numerous reports in the literature.^11,12

Overlaid contour plots (Figure 7) of crystallinity, extension at break, UTS and Young's modulus of most important factors (temperature and internal drawing ratio) allow the determination of the region corresponding to optimum conditions for polyamide 6-6 braided suture hot-stretching. The compromise region, shown in white, can be reached with the following conditions: crystallinity > 30%, UTS > 500 MPa, Young's modulus < 2.4 GPa and extension at break in the range of 20–30%. Consequently, the choice of hot-stretching temperature and internal drawing ratio should be limited to the white region, which is obtained only in two cases of hold values of residence time and take-up spool rate. In the first case (residence time = 1 min and take-up spool rate = 10.25 m.min^–1), the optimum is reached with an internal drawing ratio between 44.5% and 52% and hot-stretching temperature higher than 196℃. In the second case (residence time = 1.25 min and take-up spool rate = 11 m.min^–1), the optimum is attained with an internal drawing ratio between 44.5% and 57% and hot-stretching temperature higher than 193℃. In order to accurately ameliorate braided suture properties, a temperature higher than 196℃ is needed and the internal drawing ratio should be between 44.5% and 52%.

Fabricated braided sutures in optimum conditions with hold values of residence time (1 min) and take-up spool rate (10.25 m.min^–1) were prepared with hot-stretching temperatures equal to 220℃ and 240℃ and internal drawing of 50%. Figure 1(b) shows a hot-stretched braided suture under a temperature of 220℃. We can easily see the orientation of filaments in the direction of the braid axis and a decrease of braid diameter. The mechanical and physical properties of fabricated sutures in the optimal conditions are presented in Tables 3 and 4.

The stress–strain curves of hot-stretched braided sutures, under different temperatures illustrated in Figure 8, show that high treatment temperature (220℃ and 240℃) induces stiff structures. In fact, braided suture hot-stretched under high temperatures exhibits high initial modulus. This can be explained by macromolecular chain orientation during the hot-stretching step, which are stabilized by hydrogen bonding in the inter-connection zone between crystalline regions.

Figure 8.

Stress–strain curves of hot-stretched braided sutures under different temperatures and fixed values of residence time, take-up spool rate and internal drawing ratio.

The macromolecular chains that have the direction of the fiber will resist more strongly deformation than those having other directions. This explains the influence of orientation in the non-crystalline regions on breaking extension decrease after hot-stretching. In fact, the deformation is easier if the chains are poorly oriented. This is the case of hot-stretched braided suture under low temperatures (180℃ and 160℃).

Curves of braided polyamide sutures tested with a differential scanning calorimeter show endothermic peaks, indicating the absorption of latent heat corresponding to the polymer melting. Figure 9 shows DSC curves of hot-stretched braided sutures at different temperatures and fixed residence time (1 min), take-up spool rate (10.25 m.min^–1) and internal drawing ratio (50%). In the case of non-hot-stretched braided suture, the principal peak is obtained at 249.25℃; there is a second small peak at 255.97℃ with crystallinity of 2.11% (Table 4). The smaller peak disappears after heat treatment and only one peak appears in the case of hot- stretched braided suture under temperatures of 160℃ and 180℃. This is due to formation of new crystallites having the same average size. However, hot-stretching braided sutures under high temperatures (200℃, 220℃ and 240℃) show double melting peaks. In the literature, double melting peaks are explained by the existence of two structures with different melting behaviors. Morton and Hearle³⁶ reported the works of Hearle and Greer who prove that the first peak is explained by growth of crystallite size and the other peak corresponds to another state where many individual small crystals are formed in non-crystalline regions during polyamide annealing. Pennings et al.⁴¹ and Pennings and Zwijnenburg⁴² reported that the double peak illustrates the formation of crystals with two different sizes and this causes a melting doublet.

Figure 9.

Melting peaks shape in differential scanning calorimetry analysis of hot-stretched braided suture under different temperatures.

Conclusion

During manufacturing, polymeric fibers are formed into molecular structures having crystalline and oriented chains in the fiber direction. Although polymeric fibers are highly oriented and crystallized, amorphous regions still exist in the yarns, and these regions significantly influence the suture mechanical properties during the hot-stretching step. The obvious changes in the braided polyamide suture mechanical proprieties amend overall suture properties during tying and after implantation. For this reason, thermal treatment conditions have to be rigorously chosen during hot stretching.

The main objective of this study was to investigate physical structural changes of different polyamide fibers after hot-stretching processes were applied to braided suture in order to understand fiber morphological evolutions related to these treatments. The effects of treatment temperature, drawing ratio, residence time and take-up on tensile properties have been discussed. It was demonstrated that suture hot-stretching improves braided suture mechanical and physical properties. Temperature and drawing ratio appear as the most influential factors. Crystallinity was improved (more than 30%) after hot-stretching, but crystals growth was limited by the formation of small crystals in the amorphous region.

We note also that suture ultimate strength increases with the crystallinity. However, braided suture becomes stiffer with increasing drawing ratio. A correlation between crystallinity and mechanical properties was established and compromise regions of treatment conditions were obtained with the overlaid contour plots method. Optimal braided polyamide suture properties are obtained with hot-stretching suture at a temperature above 200℃.

The obvious changes in the polyamide structure affected braided suture mechanical properties. This amends overall braided suture properties during tying and after implantation. For this reason, thermal treatment conditions have to be rigorously chosen during hot stretching.

Footnotes

Funding

This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.

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