A new acrylamide- glyoxal-based,formaldehyde-free elastic and stiffness finishing process for silk fabric

Abstract

A new formaldehyde-free reagent based on acrylamide and glyoxal was synthesized to improve the elasticity, stiffness, and weight gain of silk fabric. The finishing process could be completed rapidly in 20 seconds. The results showed that the elasticity, stiffness, and weight gain of silk fabric were efficiently improved. The stiffness was improved from 0.03 to 0.88 N·m, the delayed crease recovery angle was increased from 240° to 288.6°, and the weight gain could reach 18.1%. The finished silk fabrics were durable. The breaking strength and tear strength were substantially increased, and the whiteness of the silk was well maintained. Scanning electron microscopy revealed that the surface of the finished silk remained smooth. Fourier transform infrared spectroscopic analysis indicated the finishing reagent reacted on the silk, and X-ray diffraction analysis indicated that a new crystalline phase formed during the finishing process.

Keywords

silk acrylicamide finish elastic stiffness formaldehyde-free weight gain

Silk is a natural protein fabric. Its chemical composition is quite similar to that of human skin. It is widely used in the apparel and household fields because of its attractive appearance, soft luster, and smooth texture. However, unlike polyester or wool fabrics, which maintain their appearance while being worn, silk exhibits poor stiffness and crease recovery.¹ Consequently, the literature contains numerous reports related to improving the crease resistance of natural silk.²

The main reasons for the formation of wrinkles are the low crystallinity and high void content of silk fibroin, and the large number of hydrogen bonds and salt bonds between the silk fibers. Under an external force, the silk fibers can slip and form hydrogen bonds in new positions. When the external force is removed, the fibers do not recover completely, resulting in wrinkle formation. Wrinkle-resistant finishes function by filling the amorphous regions with resin and inducing chemical crosslinking to prevent the silk fibers from slipping.³

Traditional wrinkle-resistant finishing primarily involves urea–formaldehyde resin and phenolic resin. For example, dihydroxymethyl ethylene urea resin (DMEU) is a crosslinking reagent that effectively increases the crease recovery of silk fabric. However, when DMEU-treated silk fabric is worn, the fabric releases formaldehyde, which is harmful to human health.^4,5

With increasing health consciousness of consumers, formaldehyde-free finishing reagents have become an inevitable development trend.^6–9 The reactivity and cost of formaldehyde-free finishing reagents are expected to be similar to those of traditional finishing reagents. New finishing reagents should exhibit low toxicity and not emit excitant odors. In addition, such reagents should not result in yellowed or damaged finished fabrics.¹⁰

Epoxy crosslinking reagents are a class of formaldehyde-free crease-resistant finishing reagents. However, their wrinkle-resistant effects are relatively poor, although the finished silk fabrics exhibit good hydrolysis stability and shrink resistance.^11,12

Polybasic carboxylic acids have attracted wide attention as finishing reagents.^13,14 An tetracarbocylic acid (BTCA) and acryloyl malic acid provide very good finishing effect among polybasic carboxylic acids developed to date. However, their prices are high, which limits industrial application. Another reagent is citric acid, which is inexpensive and nontoxic. However, its degree of crosslinking is less than that of BTCA, and it tends to result in yellowing of finished fabrics.^15–17 Guowei et al.¹⁸ synthesized a tetracarboxylic acid and a hexacarboxylic acid which also had good crease recovery effects. However, the high prices of the synthetic raw materials limit the usage of this reagent. The mechanism of the reaction of polybasic carboxylic acids with silk fabric has been studied systematically.^19–21 Compared with urea–formaldehyde resin and phenolic resin, it takes more time to complete the finishing reaction for polybasic carboxylic acids. Studies have demonstrated that the dehydration of two carboxylic acids results in the formation of an anhydride group at high temperatures, and that the anhydride groups then crosslinks with the fabric.

Glyoxal is a low-cost and formaldehyde-free finishing reagent that, under catalysis by sulfate, effectively imparts silk fabric with wrinkle-resistance and shrink-resistance properties.²² Research has shown that the effects of glyoxal as a finishing reagent are better than those of two-dimensional (2D)-resin finishing reagents,²³ and exposure to hydrochloric acid does not substantially reduce the breaking strength of the finished silk fabric.²⁴ However, glyoxal provides fewer crosslinking reaction positions for finishing with silk fibers; thus, a large amount of crosslinking reagent is required to achieve a high level of crosslinking, which results in extensive yellowing.²⁵

The weight gain finishing of silk fabric is a current research trend in silk finishing. In weight gain finishing, the mass loss of degumming can be offset to a certain extent; in addition, weight gain finishing can partially improve the stiffness of silk fabric.²⁶ As a differential fabric, stiff silk fabric can satisfy the requirements of different garments. Sericin fixation and graft polymerization are common methods of weight-gain finishing.

In this study, a three-dimensional (3D) macromolecule based on acrylamide and glyoxal was synthesized to improve both the stiffness and wrinkle resistance of silk fabrics, as well as provide a weight gain. In the finishing process, the free-radical reaction formed the backbone of the 3D macromolecule, and the polycondensation formed the branched chain. Because the free-radical reaction is very rapid, the stiffness and wrinkle resistant finishing process could be completed in only a few tens of seconds.

Experimental

Materials and chemicals

Silk fabric 12101 was purchased from a market in Chongqing, China. Acrylic amide, glyoxal aqueous solution (40%), ammonium persulfate, methanol, and aluminum sulfate were purchased from Chengdu Kelong Chemical Reagent Co. Ltd, Chengdu, China.

Synthesis of the monomer of the stiffness and wrinkle-resistance finishing reagent

Traditional resin was synthesized using formaldehyde and an amide. The reactivity of the –CH₂OH group formed by the formaldehyde and amide group was very high. The resin macromolecule rapidly formed. The reactivity of the –CHOH group formed by glyoxal and an amide group is much lower than that of the –CH₂OH group formed by formaldehyde and an amide. To prepare a formaldehyde-free resin, we used glyoxal instead of formaldehyde, and to achieve a high reaction rate, we used acrylamide as the compound with an amide group. Because acrylamide has a vinyl group, radical polymerization was induced in the resin reaction. In the synthetic process, the polymerization of the vinyl group of acrylamide formed the main macromolecular chains. Condensation polymerization among the –CHOH groups formed by formaldehyde and an amide linked the main macromolecular chains. As is shown in reaction formula (1), every production had two –CHOH groups. The polymerization and the condensation polymerization could form 3D macromolecules. Certainly, in the finishing process, the –CHOH groups could react with –OH and –NH₂ groups on the silk fibroin macromolecules. To avoid yellowing during the finishing process, the –CHOH group was etherified with methanol.

The reaction between the acrylamide and glyoxal is as follows

The reaction is a nucleophilic addition reaction.

The etherification reaction is as follows

The reaction product in (2) is called acrylamide glyoxal resin finishing reagent, it is a condensation reaction.

The acrylamide glyoxal resin finishing reagent was synthesized as follows: 15 g of acrylic amide was added to a 500-ml conical flask; 100-ml distilled water was then added to dissolve the acrylic amide, followed by the addition of 50-ml glyoxal. Sufficient water was added to adjust the solution volume to 180 ml. The conical flask was placed in a water-bath kettle, and the solution was stirred and reacted for 20 min at 50℃. Then, 20-ml methanol was added, and the mixture was continuously reacted for 40 min, yielding the monomer of the stiffness and wrinkle-resistant finishing reagent.

The percent conversion (E) of the acrylamide glyoxal resin was calculated according formula (3). A was the absorption of the acrylamide glyoxal resin solution at 263 nm, and A₀ was the absorption of the mixture solution of acrylic amide, glyoxal aqueous solution (40%) and methanol at 263 nm. The acrylic amide, methanol, and acrylamide glyoxal resin all had no absorption at 263 nm, but the glyoxal aqueous solution had a absorption peak at 263 nm.

E = \frac{A}{A_{0}} \times 100 %

(3)

The percent conversion (E) of the acrylamide glyoxal resin was 94.5%.

Stiffness and wrinkle-resistant finishing process of silk fabric

In the finishing reaction of the acrylamide glyoxal resin, there are two separate reactions, including radical polymerization and polycondensation. Ammonium persulfate was used as a catalyst in radical polymerization and aluminum sulfate was used as a catalyst in polycondensation. The finishing reaction of the acrylamide glyoxal resin was as follows To finish the silk fabric, the synthesized finishing reagent was confected into different mass concentrations ranging from 2.5% to 20%. We added 0.6-g ammonium persulfate and 0.6-g aluminum sulfate as catalysts to 30-ml finishing reagent solution and then immersed 1-g silk fabric in the finishing reagent solution at room temperature for 5 min. After immersion, the silk fabric was padded using a padder with a wet pick up of 120 ± 8%. The silk fabric was then baked at 170℃ for 20 seconds with a setting machine (Xiamen rapid co. LTD, China).

The weight gain (WG) of the finished silk fabric was calculated as follows using equation (5)

WG = (W_{1} - W_{0}) / W_{0} \times 100 %

(5)

where W₀ is the weight of original silk fabric and W₁ is the weight of the finished silk fabric.

Measurement of mechanical properties of silk fabrics

The bending rigidity of natural and finished silk fabrics was measured using a computer-controlled LLY-01 tester (Changzhou Zhengda General Textile Instrument Co. Ltd, China); the tests were conducted in accordance with standard DIN EN ISO 13934-1. The crease recovery angles of the silk fabric samples before and after finishing were tested according to standard GB/T3819 using a YG(B)541D-II automatic digitalizing crease recovery angle tester. The breaking strength and elongation of natural and finished silk fabrics were measured using an HD026N electronic fabric strength meter (Nantong Hongda, China) according to the ASTM D 5035-2006 (2008) standard for breaking force and elongation of textile fabrics (strip method). The tear strengths of natural and finished silk fabrics were measured using a tear strength tester (Dongguan huahui,China) according to GB/T3917.2-2009. The whiteness of the silk fabrics before and after finishing was determined using a Datacolor 650 (Datacolor Co., USA) with reference to the AATCC 110-2005 (2007) standard for whiteness testing of fabrics.

The durability of the finished silk fabric was evaluated by the AATCC 61-2006 standard test method using a soaping fastness tester (Roaches Co., UK). In this test, the water temperature was at 49℃. After 30 washing cycles the bending rigidity and the crease recovery angles of finished silk fabrics were measured.

Structural characterization of silk fabrics

The surface morphology of the natural and finished silk fabrics was observed using a Sirion 200 field-emission scanning electron microscope (SEM; FEI Co., The Netherlands). Before being observed by SEM, all of the silk fabric samples were coated with gold using a sputter coater.

The Fourier-transform infrared (FTIR) spectra of the natural and finished silk fabrics were measured using a spectrum GX infrared spectrometer (Perkin–Elmer Co., USA). The spectra were collected at a resolution of 1.0 cm⁻¹ in the range from 4000 to 400 cm⁻¹.

The crystalline structures of the fabrics were analyzed at room temperature using a Rigaku XD-3 wide-angle X-ray diffractometer (Beijing Purkinje General Instrument Co. Ltd, Beijing, China) equipped with a Cu K α radiation source (λ = 0.15 nm) operated at 36 kV and 20 mA. The diffraction angle 2θ was scanned from 5° to 50° in steps of 0.02°.

Results and discussion

Structural characterization of silk fabrics

Figure 1 shows the SEM micrographs of natural silk and the sample finished with 20% reagent (weight gain 19.0%). The surface of natural silk fiber is relatively smooth; the surface of the finished silk fiber was covered with some materials and there were some materials among the fibers. These materials should be the finishing reagent.

Figure 1.

SEM micrographs (×3k and ×5k) of (a, c) natural silk and (b, d) finished silk (weight gain 19.0%).

Figure 2 presents the SEM images of the cross sections of silk fibers before and after finishing with 20% finishing reagent. The finished silk fibers did not swell, and there were some materials among the fibers.

Figure 2.

SEM of the cross sectioned of (a) natural silk and (b) finished silk (weight gain 19.0%).

Figure 3 shows the FTIR spectra of acrylic amide and the synthesized acrylamide glyoxal resin. The peaks at 3350 and 3180 cm⁻¹ in the spectrum of acrylic amide are attributed to the –NH₂ group, and the peaks at 1678, 1623, and 1275 cm⁻¹ are assigned as amide I, amide II, and amide III bands. Compared to the acrylic amide spectrum, the peak at 1623 cm⁻¹ is decreased a lot; this proved the –NH₂ groups decreased a lot due to reacting with glyoxal. A new strong absorption peak at 1090 cm⁻¹ occurred, which is attributed to the C–O–C group.

Figure 3.

The FTIR spectra of acrylic amide and acrylamide glyoxal resin.

Figure 4 shows the infrared spectra of the natural silk and the sample finished with 20% finishing reagent. The peaks at 1650, 1530, and 1261 cm⁻¹ in the spectrum of the natural silk are assigned as amide I, amide II, and amide III bands, respectively. In addition to these characteristic peaks of silk in the spectrum of the finished silk, two new absorption peaks are also observed. The peak at 1126 cm⁻¹ is assigned to the asymmetrical stretching vibration of C–O–C, and the peak at 924 cm⁻¹ is attributed to the symmetrical stretching vibration of C–O–C. These peaks are consistent with the products predicted in reaction (3), and show that the finishing-reagent molecules reacted with each other and should react with silk fibroin.

Figure 4.

The FTIR spectra of natural and finished silk.

Figure 5 shows the X-ray diffraction (XRD) spectra patterns of silk before and after finished with 20% finishing reagent. The pattern of the natural silk fibers shows main characteristic diffraction peaks at 24.7° and 28.3°, which are assigned to silk I, and peaks at 9.7°, 18.6°, and 20.3°, which are assigned to Silk II. In the XRD pattern of the finished silk fibers, the peaks at 18.6° and 20.3° are more intense than the corresponding peaks in the pattern of the natural silk fibers, which indicates that the finished silk contains a greater proportion of silk II. New diffraction peaks appeared at 12.4°, 21.3°, and 35.6° in the pattern of the finished silk pattern, which shows that a new phase crystallized during the finishing process.

Figure 5.

The X-ray diffraction of natural and finished silk.

Mechanical properties of silk fabrics

Figure 6 shows the relation between the weight gain of the finished silk fabrics and the concentration of the finishing reagent. With increasing concentration of finishing reagent, the weight-gain rate of the finished silk fabric increased almost linearly. When the finishing reagent mass concentration was 20%, the weight gain of the finished silk fabric reached 19.0%. This results shows that the finishing reagent could rapidly form macromolecules, because the finishing time was only 20 seconds. (The least baking time is 20 seconds at the fastest speed in the setting machine.) The introduction of free-radical polymerization resulted in the effective formation of large molecules. This result also indicates that this finishing technology is an effective method for increasing the weight gain.

Figure 6.

The relation between weight gain and finish reagent concentration.

Figure 7 shows the relation between bending rigidity and the weight gain. Initially, the bending rigidity of the finished silk fabric increased slowly with the weight gain, and as the finishing reagent concentration continuously increased, the bending rigidity of the finished silk fabric increased substantially. This result shows that the finishing method could efficiently improve the bending rigidity, because the finishing reagent formed 3D macromolecules, resulting in the dramatic improvement of bending rigidity of the finished silk fabric.

Figure 7.

The relation between bending rigidity and weight gain.

Figure 8 shows the relation between the crease recovery angles and the weight gain of the silk fabric. As shown in this figure, the crease recovery angles are substantially improved with the increasing weight gain. The acute crease recovery angle of the natural silk fabric was approximately 187°; after the finishing process, the crease recovery angle reached 247.1°. The delayed crease recovery angle of natural silk fabric was approximately 240°; after the finishing process, the crease recovery angle increased to 288.6°. These results demonstrated that the elasticity of the silk fabric could be effectively improved by the finishing reagent. The improvement of the elasticity is a consequence of most of the finishing reagent seeping into the silk, a part of the reagent forming a 3D polymer during the finishing process. When the silk fabric was subjected to an external force, fiber slip was very difficult. Thus, the elasticity of the silk fabric was dramatically improved.

Figure 8.

The relation between crease recovery angles and weight gain.

Figure 9 shows the relation between the weight gain and the breaking strength. The breaking strength of the silk fabric increased steadily with the weight gain, because the finishing reagent reacted to form a 3D polymer, resulting in an increase in the breaking strength of the finished silk fabric. These results demonstrate that the finishing process strengthened the silk fabric.

Figure 9.

The relation between the weight gain and breaking strength.

Figure 10 shows the relation between the breaking elongation and the weight gain. The breaking elongation of the silk fabric decreased a little with the weight gain. This is because the finishing reagent filled in the space between the fibers.

Figure 10.

The relation between the breaking elongation and the weight gain.

Figure 11 shows the relation between the tear strength and the weight gain. With the increase of weight gain, the tear strength of the silk fabric increased a little. This is in accordance with the breaking strength.

Figure 11.

The relation between the weight gain and tear strength.

Figure 12 shows the relation between the weight gain and the whiteness of the silk fabric. The whiteness of the finished silk fabric gradually decreased with increasing weight gain; however, the whiteness only decreased from 88.09 to 84.81. Thus, the finished silk fabric appeared to well-maintain its whiteness. This result indicates that the methoxylation of the hydroxyl groups in the finishing reagent can prevent the yellowing phenomenon of silk fabrics.

Figure 12.

The relation between whiteness and weight gain.

Figure 13 shows the relationship between bending rigidity of finished silk fabric (weight regain 11.0%) and washing times. From Figure 10, the bending rigidity of finished silk fabric decreased a little with the increase of washing time. After 15 times, the bending rigidity barely decreased further with the increase of washing times. The bending rigidity was well sustained.

Figure 13.

The relationship between bending rigidity of finished silk and washing times.

Figure 14 is the relationship between crease recovery angle of finished silk fabric and washing times. From Figure 14, the acute crease recovery angle decreased more than the delayed recovery angle. But the crease recovery angle only decreased a little; the folding and elastic property of the finished silk fabric was well maintained after washing.

Figure 14.

The relationship between crease recovery angles of finished silk and washing times.

Conclusions

A novelty monomer finishing reagent based on acrylic amide and glyoxal was synthesized. This finishing agent could greatly improve the elasticity, stiffness, and weight gain of silk fabric. The whiteness of the silk fabric was well maintained, and its breaking strength was substantially increased by the finishing process. The finishing process was completed rapidly via a free-radical polymerization and condensation reaction; it only needed 20 seconds to complete the finishing process. The SEM showed there were some materials among the silk fibers, and the FTIR spectra and X-ray diffraction indicated the finishing reagent reacted on the silk.

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: This study was supported by the Chinese National Undergraduate Innovation Program (grant number 201410635012).

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