PP fiber is a semi-crystalline thermoplastic polymer, widely applied in decoration cloth, packing fabric, non-woven fabrics, workwear and medical care dressing due to its excellent mechanical properties and good chemical stability. But PP fiber has no polar group on its surface, which makes it difficult to be dyed on the one hand, and static electricity easy to generate on the other hand [1]. Thus, it is very necessary to pursue the hydrophilic modification of PP fiber. Universal hydrophilic modification of PP involves surface coating [2, 3], wet chemical [4,5,6] and physical radiation containing plasma discharge [7,8,9,10]. It is reported that low temperature plasma treatment can positively produce large amounts of the polar group on the PP surface [11, 12]. However, the surface properties improved by plasma usually weakens with the lay-up time, known as the ageing effect of plasma treatment, because the polar groups produced will decay [13]. To attain lasting hydrophilicity, the grafting polymerisation of hydrophilic monomer is developed after plasma treatment [14,15,16,17]. However, in this method it is difficult to control the grafting rate [18, 19].
Actually, when many polar groups are produced on the surface of PP fiber after plasma treatment, the fibers can attach hydrophilic agents, aided by crosslinkers. Bratskaya et al. described the coating of chitosan onto O2-plasma treated PP surfaces through epichlorohydrin as the crosslinker [20]. Tsou et al. [21] reported quaternary methyl diallyl ammonium salt grafting onto O2 plasma treated PP nonwovens; the finished PP fabric indicated favourable antibacterial and hydrophilic properties. Wu et al. [22] introduced a modification of polypropylene treated by air plasma and feather keratin graft, the results of which showed that the surface hydrophilicity and printing property were greatly improved. Ma et al. [23] achieved three hydrophilic biomolecule grafting onto PP fabric using NH3 and N2 low temperature plasma pre-treatment.
As is known, polyols possess good hydrophilicity due to its multi-hydroxy groups, including sorbitol, maltitol and polyethylene glycol. Sorbitol is an important food humectant and has been gradually applied in pharmacology, cosmetics, and other fields [24,25,26]. Maltitol, a disaccharide consisting of a glucose unit linked to a sorbitol one via an α-1,4 bond, is obtained from maltose by hydrogenation. It is mostly used in food, toothpastes, and tablets owing to its high affinity for water, low calorie sweetener and effective acaricidal activity [27, 28]. PEG is a polymer from ethylene glycol, which exhibits excellent moisture retention, adhesive, and antistatic properties in many fields [29,30,31]. Once polyols are linked to the surface of the plasma-treated PP fabric, permanent hydrophilicity can be obtained.
For polyol grafting, a crosslinker which can react with the hydroxyl group is necessary. N-methylol reagents (DMDHEU), polycarboxylic acids (BTCA), acrylamide (FAP or MBA), blocked-diisocyanate (HDACS), and divinyl sulfonyl sulfate are effective reagents and have been well demonstrated in cotton or cellulose fiber crosslinking [32,33,34,35,36,37,38]. However, formaldehyde will be released when DMDHEU-treated fabric is subjected to multiple laundering cycles, and the reaction of other crosslinkers needs high temperature. Considering saving energy, low temperature crosslinking is the most desirable. Epoxy and aziridine crosslinkers were reported to react at low temperatures [39, 40], due to the three-membered heterocyclic ring at the less substituted carbon atom, which is susceptible to attack by a range of nucleophiles (e.g., amines, alcohols, acids) [41, 42]. Kurmaev et al. [43] used X-ray fluorescence measurements to show that the epoxy group of EGDE as a crosslinker reacted with the hydroxyl group of chitosan. Xiao-Qiong Li and Ren-Cheng Tang [44] analysed the FTIR spectra of crosslinked chitosan fiber with an SaC-100 crosslinker, implying that the hydroxyl groups in chitosan react with the aziridine.
Based on the above investigation, this study aimed to graft polyols (sorbitol, maltitol, polyethylene glycol) onto the surface of PP to develop a hydrophilic and antistatic fabric. To provide a grafting site for PP, an O2 plasma pretreatment was firstly designed to produce – COOH or -OH. Then, the attachment of polyols on the surface of the fiber was performed using EGDE and TTMA, respectively. Finally, the wettability, moisture retention, antistatic and coloring performance of modified PP fabric were determined and analysed.
The PP woven fabric (warp/weft: 182/96, 364 g/m2) used in this study was supplied by Anping Country Yongding Filter Cloth Weaving Co., Ltd (China). The samples (4cm×8cm) were cleaned with acetone and distilled water in an ultrasonic cleaning machine before the experiment to remove impurities. Three kinds of polyols (sorbitol, maltitol and PEG-800) were purchased from Tianjin Fengchuan Reagent Technology Co., Ltd (China). EGDE and TTMA crosslinkers were purchased from Chengdu Aikeda Chemical Reagent Co., Ltd (China).
Structure diagram of polyols (sorbitol, maltitol, PEG) and crosslinkers (EGDE, TTMA)
The whole experiment was divided into two parts: plasma pretreatment of PP fibers and polyols post-grafting.
Flow chart of plasma treatment and polyol modification
Plasma technology is a new modification technology for fiber and textile which can improve the hygroscopicity of fiber and the dyeing property of textile. Depending on the type of dopant used, two main classes of treatments can be distinguished: grafting and coating. This paper mainly adopted the grafting way to activate and etch the sample surface [45].
PP fabric (4cm×8cm) was treated on glow-discharge plasma apparatus (Omega DT-03), which was supplied by Suzhou Optics Plasma Co., Ltd (China). Oxygen was used in the plasma pretreatment. The electrode distance was 4.5cm. The entry flow (200sccm, 250sccm, 300sccm), discharge power (50W, 100W, 150W, 200W, 250W, 300W), and treatment time (90s, 120s, 150s, 180s, 210s, 240s) were set as variables for the experiment.
Three 5wt% polyol solutions (sorbitol, maltitol, PEG-800) were individually prepared by dissolving the crosslinker in distilled water (100mL). The pH value was adjusted to 7–8 for the EGDE solution, and the pH value was 6–7 for the TTMA solution. Then the plasma-treated PP fabric was dipped into the polyol solution for a grafting effect. It was a single factor variable experiment, whose variables were the amount of crosslinkers (2%, 4%, 6%, 8%, 10%) (o.w.f), temperature (AMB, 50°C, 80°C), and time (8h, 12h, 16h, 20h, 24h). After the experiment, the samples were heated at 120°C for 3min, then rinsed with water, and finally dried again.
The surface chemical composition of the samples was determined using a Fourier transform infrared spectrometer (IR affinity, Shimadzu, Japan). The KBr tablet method was used for sample preparation. The samples were cut up and ground in a mortar with KBr, then placed in a press for tablet pressing. The samples were then put onto the spectrometer for testing. The scanning range was 500 ~ 4000cm−1, the number of scans 200, and the resolution was 4cm−1.
Reactive dyes (reactive yellow KE-4R, reactive scarlet B-3G, reactive deep blue K-R) were used to dye the PP fabrics at a bath ratio of 1:50. The PP fabric was dyed at 60°C for 30 minutes, and NaCl and Na2CO3 were added. After that, the PP fabric was dyed at 90°C for 30 minutes, then washed and dried. The dye-uptake was calculated by
The grafting rate was used to analyse the immobilisation results of the polyols.
An S-3400N scanning electron microscope was used to observe the surface morphology changes of the original sample, the sample after plasma treatment, and the sample after modification. Before the SEM observation, the samples were coated with a carbon film, and then with a gold film (5nm) by ion sputtering. The voltage was 20kV, the working distance 11.8 mm, and the magnification was 2000x.
Loose fiber was suspended in H2O to test the hydrophilicity of PP fabrics. First of all, the test sample is separated into fibers and marked. After that, the fiber was put in a beaker with 100mL of water for 3 min, and then the fiber settling velocity was observed and recorded. To avoid contingency, each group was tested three times.
The level of moisture release of PP fiber was used to characterise its moisture retention. The samples were firstly completely dried, and then they were placed in a humidity chamber under normal conditions (25 °C, 65% RH) and high humidity conditions (30 °C, 95% RH) successively for 80min. Lastly, the samples treated with high humidity were taken out and put into normal conditions (25 °C, 65% RH) again for 160 min. The sample was weighed for a random period of time. Moisture regain is defined by
The antistatic performance of the modified PP was analysed according to the half-life value. The samples (6cm×8cm) were treated by a fabric static tester (YG (L) 342D, Shandong Textile Research Institute (China)). The test conditions were 20 °C, and the relative humidity was 35%. The sample was placed in a high voltage electrostatic field when the charge was stable, and then the voltage was naturally attenuated through a grounding metal station. When the voltage value had decayed to the half, the corresponding time was recorded.
The more O-containing groups produced on the PP surface after O2 plasma pre-treatment [23, 46], the more favorable polyol post-grafting is. Hence, it is necessary to optimise the plasma treatment of PP fiber.
The original PP lacks polar functional groups, but the chemical composition of its surface will change after plasma treatment.
Infrared spectra of PP before and after O2 plasma
Absorption intensity of the characteristic peak and colouring performance of PP fabric after plasma treatment
Factors | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Power (W) | 50 | 100 | 150 | 200 | 250 | 300 | 250 | 250 | 250 | 250 | 250 | 250 | 250 |
Time (s) | 90 | 90 | 90 | 90 | 90 | 90 | 120 | 150 | 180 | 210 | 240 | 180 | 180 |
Flow (sccm) | 300 | 300 | 300 | 300 | 300 | 300 | 300 | 300 | 300 | 300 | 300 | 250 | 200 |
O-H (3340 cm−1) | 0.10 | 0.10 | 0.10 | 0.11 | 0.20 | 0.14 | 0.26 | 0.32 | 0.42 | 0.28 | 0.22 | 0.23 | 0.15 |
C=O (1716 cm−1) | 0.13 | 0.13 | 0.13 | 0.18 | 0.28 | 0.22 | 0.37 | 0.41 | 0.50 | 0.39 | 0.24 | 0.31 | 0.19 |
Dye uptake (%) | 12 | 15 | 18 | 25 | 30 | 28 | 32 | 33 | 37 | 29 | 27 | 26 | 21 |
K/S value | 0.24 | 0.37 | 0.51 | 0.68 | 0.65 | 0.72 | 0.84 | 0.89 | 1.03 | 0.88 | 0.77 | 0.63 | 0.49 |
The effect of O2 plasma treatment on PP fiber can also be evaluated by a colouring experiment with reactive dyes because the reactive groups of dyes react with the O-containing groups (-OH or -COOH) of plasma (O2)-PP fibers in the form of covalent bonds.
Colouring photos of PP fabric before and after O2 plasma treatment: a-c) original PP fabric, d-f) plasma (O2)-PP fabric
The single factor variable experiment was used to analyse optimal polyol grafting onto plasma (O2)-PP fabric.
Influence of different factor levels on the grafting rate: a) the influence of temperature on the grafting rate (12h, 4% crosslinker), b) the influence of time on the grafting rate (EGDE: 80°C, 4% crosslinker. TTMA:50°C, 4% crosslinker.), and c) the influence of the amount of crosslinker (EGDE: 80°C, 16h. TTMA:50°C, 12h.)
To investigate the grafting effect at low temperature and to prevent the self-polymerisation of polyols at high temperature, 80°C was set as the upper limit of grafting. The effect of the reaction temperature on the grafting rate was investigated under the condition of 4% crosslinker for 12h. Clearly, the grafting rate increased with the increase of temperature (
Under conditions of 80°C, 4% EGDE crosslinker usage, and 50°C, 4% crosslinker usage, the influence of time on the grafting rate is investigated. The corresponding result is presented in
Polyols grafting experiments were carried out with various amounts of the crosslinker under conditions of 16h at 80°C for EGDE, and 12h at 50°C for TTMA.
The surface morphology of PP fabric will change after plasma treatment and grafting modification; thus, the surface morphology of PP fabric was observed in three stages by scanning electron microscope.
SEM images of the three finishing stages of PP fabric: a) untreated PP, b) PP treated by oxygen plasma, and c) PP grafted with PEG
The hydrophilicity change of the PP fibers was investigated by a water immersion experiment of loose fibers.
Loose fiber suspension picture with different finishing methods: a) untreated PP, b) PP treated by oxygen plasma, c) PP grafted with sorbitol, d) PP grafted with maltitol, and e) PP grafted with PEG.
Moisture retention of fibers determines comfort in application.
Changes in moisture absorption and desorption of untreated PP and grafted PP
The antistatic property of PP fabric was tested by the half-life method.
Electrostatic pressure peak value and half-life period of PP fabric with different finishing methods
Samples | Electrostatic pressure peak value [V] | Half-life period [s] |
---|---|---|
Untreated | 1102 | >99 |
PP-EGDE-Sorbitol | 507 | 25 |
PP-EGDE-Maltitol | 485 | 22 |
PP-EGDE-PEG | 463 | 17 |
PP-TTMA-Sorbitol | 405 | 14 |
PP-TTMA-Maltitol | 389 | 13 |
PP-TTMA-PEG | 342 | 9s |
The half-life period can represent the time of charge loss; hence, the shorter the half-life, the better the antistatic performance [50]. The untreated PP fabric is not antistatic according to the data. After polyol grafting, the electrostatic voltage and half-life of PP decreased sharply, indicating the finished fabric possessed antistatic ability, among which PEG-PP fabric has the best. Besides this, the polyols-PP fabric obtained with a TTMA crosslinker had stronger antistatic ability than that with EGDE crosslinker, as a result of the higher grafting ratio.
Polypropylene fabric was modified with plasma pretreatment and polyol post-grafting. The modified PP fabric has hydrophilic, colouring and antistatic properties. Some conclusions can be drawn based on the results:
The optimal process (250W, 180s and 300sccm) of O2 plasma treatment on PP fabric was obtained by evaluation of the IR characteristic peak and dyeing effect of experimental samples. With EGDE or TTMA crosslinking, three polyols could be successfully grafted onto plasma (O2)-treated PP fiber, but the effect using TTMA as a crosslinker was better than for EGDE. The grafting ratio order of PEG>maltitol>sorbitol was maintained, regardless which crosslinker was used. The optimal grafting process, 16h at 80°C for 8% EGDE and 12h at 50°C for 6% TTMA were obtained respectively. After polyol grafting, the PP fabric possessed distinct water immersion, moisture retention and antistatic ability.
The above shows that the experiment of grafting polyols onto PP fabric after oxygen plasma treatment is feasible, which can improve the hydrophilic and antistatic properties of PP fabric.