Abstract: In this study, we used citric acid (CA) as a crosslinking agent, mixed with polyhexamethylene biguanide, to perform a pad–dry–cure treatment on wool fabrics to study its antimicrobial effects and physical properties. The surface characteristic and the structure of wool fabric were investigated by means of scanning electron microscopy (SEM). Fourier transform infrared spectroscopy (FTIR) was employed to study the crosslinking mechanism of the treated fabric. The concentration of finishing agent polyhexamethylene biguanide in the presence of citric acid as well as the treatment conditions significantly affected the performance properties activity of treated wool fabrics. The treated wool fabrics showed good antimicrobial activity. From the experimental results, we learned that polyhexamethylene biguanide and CA did not crosslink with the wool fibers if the wool fabrics were not oxidized by hydrogen peroxide and that after oxidization, CA produced esterification with the -OH group of the wool and transamidation with the NH2 group of the polyhexamethylene biguanide to form a crosslink. The surface crosslink of the oxidized wool fibers were relatively coarse, which beneficial for the antimicrobial and antiseptic effects of the wool fabrics.
Key Words: Polyhexamethylene biguanide; antimicrobial finishing; wool; citric acid; crosslinking
Application of natural antimicrobial agents on textiles dates back to antiquity, when the ancient Egyptians used spices and herbs to preserve mummy wraps. Natural antimicrobials were used to inhibit the growth of bacteria and mould in the fabric. The prevention of microbial attack on textiles has become increasingly important to consumers and textile producers; therefore, interests in antimicrobial fabric finishing have steadily increased over the last few years.
Antimicrobial textiles are classified as those textile and fibrous materials subjected to various finishing techniques to afford protection for both the user of textile materials (against bacteria, yeast, dermatophytic fungi, and other related micro organisms for aesthetic, hygienic or medical purposes) and the textile itself (biodeterioration caused by mould, mildew and rot producing fungi) without negatively affecting the other important characteristics of the textiles. Commercially available polymer, polyhexamethylene biguanide has many chemical attributes to make it an interesting candidate for antimicrobial applications[1-2].
Poly (carboxylic acid) (PCA) is quite effective for antiwrinkle treatments for cotton fabric; by replacing the conventional formaldehyde-type treatment resin, PCA forms a five-membered cyclic anhydride intermediately in the presence of a catalyst and then produces an ester bond with R-OH. The many hydroxyl bonds (e.g., serine, tyrosine) on the wool fiber structure can form ester bonds with PCA and crosslink. Among the non-formaldehyde-type treatment resins for the esterification crosslinking of cotton fabric, PCA, BTCA (1,2,3,4-butane tetracarboxylic acid), and CA are the most favorable compounds. Although BTCA is very effective, it is very costly. Thus, by reference, we learned that CA is a feasible crosslinking agent .
Wool is a natural protein fiber that has been widely used as a high-quality textile material. For antimicrobial treatment of wool fabrics, in addition to conventional methods, such as organic silicon quaternary ammonium compounds, halogenated diphenyl ether derivatives, nitrofurantoin, and organic nitrogen compounds, research on the antimicrobial treatment of wool fabrics by PCA crosslinking agents and polyhexamethylene biguanide is scarce. Thus, this study was devised to use citric acid (CA) as a crosslinking agent to study the antimicrobial treatment of wool fabrics through the mixture of CA with polyhexamethylene biguanide. In this study, the influences of polyhexamethylene biguanide in the presence of CA as well as the finishing conditions on the performance properties were studied, the surface characteristic and the structure change of wool fabric treated was investigated by means of scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) was used to study the reaction mechanism during crosslinking. Wash fast and antimicrobial tests were also conducted to evaluate the wool fabric antimicrobial treatment. At the same time, changes in the breaking strength, whiteness, and wrinkle recovery angles of the treated object were also investigated to study the changes in the physical properties of the wool fabric during heat treatment.
The experimental wool fabric had a warp density 178, a weft density of 86.The fabric used was a weave 100% wool fabric. Polyhexamethylene biguanide, CA and hydrogen peroxide were purchased from Saintyear Holding Group Company (China). Hypophosphite, sodium silicate and sodium carbonate were all first-grade reagents.
2.2. Fabrics treatment
① Hydrogen peroxide pretreatment procedure
Prior to antimicrobial treatment, all wool fabrics were pretreated for 1h at 70℃ in baths containing 2g/L of sodium silicate and 18g/L of hydrogen peroxide (30%) with a 25:1 liquor-to-goods ratio, pH: 9, then thoroughly rinsed in distilled water and air dried.
② Antimicrobial treatment
The wool fabrics were padded two dips and nips (100% wet pick up) in a solution containing polyhexamethylene biguanide, citric acid (CA) and hypophosphite. After treatment, the wool fabrics were dried at 80℃ for 5 min and cured, then thoroughly rinsed in 50℃ hot water and air dried in a standard atmosphere for testing (20±1℃ and 65±2RH).
2.3. Testing and analysis
The wrinkle recovery angle (WRA) was measured according to AATCC test method 66-2003 and tensile strength (TS) according to ASTM D5034. Whiteness was evaluated from the Datacolor SF600 color measuring and matching instrument. The antimicrobial test was carried out according to the antimicrobial standard of the Japan Association for the Evaluation of Textile and was measured according to quantification methods JIS L1902-2002. Surface morphological structure of untreated and treated wool fabric was measured with a SM-5600LV scanning electron microscopy (SEM). Fourier transform infrared spectroscopy (FTIR) was employed to study the crosslinking mechanism of the treated fabric. The durability of the treated wool fabric against repeated launderings was evaluated by washing wool fabrics according to AATCC test method 124.
3. Results and discussion
3.1. Effect of polyhexamethylene biguanide concentration
The effect of polyhexamethylene biguanide concentration on the performance properties of the treated fabrics is shown in Table 1.
Table 1 shows that increasing the amount of polyhexamethylene biguanide in the finishing bath is accompanied by a decrease in the tensile strength and WI. The loss of tensile strength, and WI, may be due to the more crosslinking behavior of polyhexamethylene biguanide with wool fabric at higher concentration. The values of WRA increase by increasing the polyhexamethylene biguanide concentration from 0 to 1%, then decrease by further increase in the polyhexamethylene biguanide concentration. The loss of WRA treated fabrics at higher polyhexamethylene biguanide concentration owing to the reaction of finishing agents with CA rather than crosslinking of wool.
3.2. Effect of finishing citric acid concentration
Table 2 shows the effect of crosslinking agent CA concentration on the performance properties of the treated wool fabrics.
The wrinkle recovery angle (WRA) increased by increasing the crosslinking agent CA concentration. The enhancement in the WRA is acceptable, since increasing the CA concentration will increase the availability of crosslinking molecules and consequently increase its accessibility to crosslink the wool hydroxyls. On the other hand, increasing the crosslinking agent concentration leads to a decrement in the values of tensile strength (TS) and whiteness index (WI). The decrement in the tensile strength is due to the hydrolytic action of the crosslinking agent at higher temperature. The decrease in the WI of the treated wool fabric may results primarily from the use of citric acid. CA decomposes at elevated temperatures to acids with double bonds, which are the primary cause of this detrimental effect. Also the crosslinking reaction may cause an additional yellowing for the treated fabrics.
3.3. Effect of curing temperature and time
Improvement in textile properties of fabrics finished with polyhexamethylene biguanide antimicrobial agents depends greatly upon the curing temperature and time. Table 3 and Table 4 shows the performance properties of the treated wool fabrics cured at different temperature and time.
In general, at higher cure temperatures much longer times were needed to increase the WRA of the finished fabrics. The curing temperature and time was inversely proportional to tensile strength, WI The increment in the WRA may be attributed to higher temperature and longer times as well as greater availability of crosslinking agent CA molecules to crosslink wool. While the loss of TS and WI reflects the hydrolytic effect of the crosslinking agent CA. Increasing the curing temperature and/or prolonging the curing time improve the extent of crosslinking of crosslinking agent CA to Wool and increase of the linked Polyhexamethylene biguanide.
3.4 The optimal technical conditions experiment
The optimal technical conditions: Polyhexamethylene biguanide, 1%; CA, 1%,Hypophosphite, 1%; Curing temp, 120℃ for 2min; Drying temp, 80℃ for 5min. Wool fabric was treated by above the optimal technical conditions. The performance properties of the treated wool fabrics were shown in Table 5.
Table 5 shows wool fabric treated with polyhexamethylene biguanide and CA had a slightly enhanced wrinkle recovery angle (WRA), Whiteness index (WI) was obvious increased, which is related to the pretreatment of hydrogen peroxide. Tensile strength (TS) was marginally decreased.
3.5 SEM analysis
The surface characteristic and the structure change of wool fabric treated was investigated by means of scanning electron microscopy, surface morphological structure of untreated wool and treated wool were shown in Fig.1 and Fig.2.
Fig.2 showed that hydrogen peroxide pretreatment has a light damage on surface scalelike structure of wool compared to untreated wool fabric (Fig.1), which is the result of oxidation and scission of the numerous disulfide bonds in the exocuticle of the wool.
3.6 FTIR analysis
Wool is composed of a cuticle and cortex and a medulla only in the case of course wool. The cuticle has valine, disulfide bond groups and carboxyl groups, whereas the cortex makes up the main portion of the wool. It is made up of more than 18 amino acids, which can be divided into four distinct groups: cationic, anionic, nonpolar, and polar. The main functional groups include carboxyl (-COOH), amino (-NH2), and hydroxyl (-OH) groups; thus, the chemical properties were extremely active. In IR spec-troscopy, the main characteristic appeared between 1000 and 1700 cm-1, including amide I (1644 cm-1), amide II (1535 cm-1) and amide III (1239 cm-1). C-O contraction appeared between 1000 and 1300 cm-1[4-5]. FTIR curve of untreated wool fabric was shown in Fig.3.
Fig.3. FTIR spectra of the untreated wool
PCA, after the introduction of the catalyst (NaH2PO2. H2O), becomes a five-membered cyclic anhydride intermediate. To form a cyclic anhydride, there must be a pair of adjacent -COOH groups. After the cyclic anhydride is formed, it produces esterification with the -OH on wool (or the chitosan -OH), at the same time releasing a -COOH group, but if the middle position (a-OH, 2-OH) carboxyl produces esterification, then it cannot form a five-membered cyclic anhydride intermediate and, thus, forfeits its activity. If it forms an ester and not in the middle, there are still two carboxyl groups able to form another cyclic anhydride; thus, it can produce bonding with the wool -OH (or chitosan -OH) again. CA can undergo esterification with the wool -OH group and can form transamidation with -NH2 and crosslink. The reaction mechanism is as follows:
R —COOH + HO —Wool →R —CO •O —Wool
R —COOH + H2N —Wool →R —CO •NH —Wool
Where R is the CA backbone and W is the wool backbone. The wool fabric was treated with polyhexamethylene biguanide and CA without a hydrogen peroxide pretreatment. The result was displayed in Fig.4.
Fig.4. FTIR spectra of wool fabrics treated with polyhexamethylene biguanide and CA
Fig.4 shows that its spectroscopic data was similar to untreated wool fabric (Fig.3); no reaction occurred. This was caused by the cuticle and rigid scales on the surface of wool fiber; when the fabric was oxidized with hydrogen peroxide and then treated with polyhexamethylene biguanide and CA, the results are displayed in Fig.5.
Fig.5. FTIR spectra of wool fabrics oxidized with hydrogen peroxide and then treated with polyhexamethylene biguanide and CA
Fig.5 shows that at 1233 cm-1, there was C-N contraction, which is shown later compared to untreated wool, C-O contraction appeared between 1041 and 1078 cm-1, which meant that the PCA -OOH group and -OH, -NH2 groups of the wool fiber became –COO- and –CONH- increased; in other words, there was an increase in crosslinking. The CA -COOH group can also undergo esterification and transamidation with the -OH, and -NH2 groups of polyhexamethylene biguanide as follows:
R—COOH + HO —Guanidine→R—CO •O —Guanidine
R—COOH + H2N —Guanidine→R—CO •NH —Guanidine
3.7. Antimicrobial and wash fast analysis
For wool fabric, according to the Japan Association for the Evaluation of Textile antimicrobial standard, abacteriostatic value greater than 2.0 means that the test sample is bacteriostasic, and an antiseptic value greater than 0 means that the sample possesses antiseptic effects. Wool fabric under various conditions of heat treatment was wash-tested multiple times according to AATCC test method 124。The antimicrobial and wash fast results are illustrated in Table 6.
The wool fabric first underwent a hydrogen peroxide preprocess, which caused the scale to oxidize and resulted in regional damage of the scale and increased the crosslinking reaction. These were then treated with polyhexamethylene biguanide and CA. Table 6 shows that because the amino groups of the polyhexamethylene biguanide in the treatment agent easily formed an amine salt cationic, which could catch the anionic bacteria and cause its cell wall to stop growing, it showed good antimicrobial properties.
The antimicrobial nature decreased with increasing number of washings. The more the washings there were, the more polyhexamethylene biguanide detached from the wool fabric, and its antimicrobial effect decreased, but after 20 washings, a bacteriostatic value was 2.7 an antiseptic value was 0.2, it maintained decent antimicrobial properties, revealing a good crosslinking effect.
Wool fabrics were treated with polyhexamethylene biguanide in the presence of crosslinking agent CA to provide the wool fabrics a good antimicrobial property by chemical linking of polyhexamethylene biguanide to the wool. Both concentrations of polyhexamethylene biguanide and CA as well as the treatment conditions significantly affected the performance properties of treated wool fabrics. The maximum performance properties was obtained when the wool fabric were treated with 1% polyhexamethylene biguanide, 1% CA, and cured at 120℃ for 2min. Treated wool fabric had a slightly enhanced wrinkle recovery angle (WRA), Whiteness index (WI) was obvious increased. Tensile strength (TS) was marginally decreased. The treated wool fabrics showed good antimicrobial activity and good durable wash fastness, after 20 washings, a bacteriostatic value was 2.7 an antiseptic value was 0.2. FTIR spectra show that because of the scale, when the wool fabric was treated with CA and polyhexamethylene biguanide solution, no crosslinking was produced. After the wool fabric was oxidized by preprocessing with hydrogen peroxide and then treated with the CA and polyhexamethylene biguanide solution, esterification and transamidation were produced.
ZHAO Xue1, HE JIN-xin1, ZHAN Yi-zhen2