Patterned Printing of Fragrant Microcapsules to Cotton Fabric

Abstract

Microcapsules with fragrance in the core were used to produce a scented textile. The wall of microcapsules was based on a melamine-formaldehyde polymer while an essential oil was in the core. They were applied to cotton fabric using screen printing in two ways: over the entire surface and by pattern. The properties of the differently printed samples were analyzed. The fragrance evaluation was performed, mechanical properties were studied, antibacterial activity against Staphylococcus aureus and Escherihia coli was evaluated, and resistance of the samples to soil microorganisms was determined. The amount of formaldehyde on the samples was measured. The results showed that all samples kept the fragrance even after 10 washes. The mechanical properties of the fully printed fabric were different from the properties of the patterned fabric. None of the prints of scented microcapsules provided antibacterial activity. All samples were biodegradable. Less formaldehyde was measured on patterned samples than on fully printed samples. The amount decreased after washing.

1. Introduction

Applying fragrance to textiles can increase the worth of various products, such as clothing, footwear, decorative textiles, some fabrics intended for a technical application, etc. [1,2]. The fragrances used for finishing are mostly essential oils, which are useful not only for their pleasant smell but also for their multiple therapeutic effects [3]. Some of them have an invigorating effect, while others have a calming, disinfecting, pain-relieving, hormone-regulating effect, etc. [4]. However, since fragrances and essential oils are volatile substances that quickly evaporate during washing, the most important question in the preparation of scented textiles is how to prolong their lifetime of scent [5,6]. One of the methods used to apply fragrances to textiles to avoid the problem of volatility and durability is microencapsulation. This is a technology in which various active agents, such as perfumes, repellents, flame retardants, dyes, etc. are packaged in small, sealed capsules (microcapsules) for controlled release under specific conditions [7,8]. In other words, microencapsulation is a low-cost method to store active ingredients for a long time [9]. The active compounds are protected inside the capsules (permeable or impermeable) from the environment and all degrading factors (light, moisture, oxygen). During washing cycles and wearing, some microcapsules break as a consequence of external force (rubbing) and gradually release the fragrance [10].

There are many possibilities for the physical application of microcapsules to textiles. Screen printing is just one of them [11]. The advantage of this technique is that microcapsules can be applied to the entire fabric or only to specific areas of a fabric or textile product. Good distribution and deposition control of the capsules also favor screen printing, as shown in one of our previous papers comparing screen printing and pad techniques [12]. When the diameter of the microcapsules is below 10 μm, they can be treated as pigments and can be bound to textile yarns with standard pigment binders. For this reason, they are resistant to multiple washings. In addition to printing and padding, coating, spraying, or bath exhaustion can also be used to apply microcapsules to textiles [13,14,15,16]. Each technique has its advantages and disadvantages.

In this study, microcapsules which consisted of an impermeable melamine-formaldehyde wall and a core of sage, lavender and rosemary essential oil were used to prepare scented textile fabric. Our goal was to apply microcapsules with screen printing in two ways: over the entire surface and by pattern. The goal was to compare the analysis results of the differently printed samples and find out if both methods are suitable for effective fragrance treatment of cotton, or better if a patterned product offers the same benefits as a fully printed one. We assumed that with patterned fabrics we would get even better results in mechanical properties, the same presence of fragrance after washing and lower amounts of free formaldehyde than with fully printed fabrics.

2. Materials and Methods

2.1. Materials

We used fully bleached and mercerized 100% cotton woven fabric (plain weave, 124 g/m², warp density 55 threads/cm, weft density 29 threads/cm) from Tekstina d.d. (Ajdovščina, Slovenia). Aero, Chemical, Graphic and Paper Manufacturers d.d., Celje, Slovenia prepared a 31% aqueous suspension of microcapsules (MC) by an in situ polymerization method. The MC had a pressure-sensitive melamine–formaldehyde polymer wall and a liquid core of 20% essential oil of lavender, rosemary and sage (1:2:7), The suspension of microcapsules and its micrograph is presented in Figure 1. Tubivis DRL 300, a synthetic polyacrylate thickener and Tubifast AS 30, a polyacrylate binder were obtained from CHT (Tubingen, Germany). A pigment, Bezaprint Gruen BT, was a product of Bezema (Montlingen, Switzerland).

The size distribution of microcapsules was measured with a laser granulometer Alcatel 715 (CILAS, Orleans, France). The distribution was narrow, with average diameters from 4 to 8 μm. Some particles smaller than 2 μm were found in the suspension, which were probably a residue of unreacted wall material from the encapsulation process.

2.2. Printing of Cotton Fabric

The recipe for the printing paste is presented in

Table 1. The recipe of the printing paste.

Component	Quantity (g/kg)
Thickener	34
Binder	150
Pigment	2
Suspension of MC	100
Distilled water	714

. The paste viscosity was 12.6 Pa·s at shear rate 2 s⁻¹, measured with viscometer Rheolab QC (Anton Paar, Ostfildern, Germany). Mini MDF R-390 (Johannes Zimmer AG, Klagenfurt, Austria) laboratory magnetic flat-screen printing machine was used to print the fabrics.

Printing was performed in two ways: on the entire surface of the fabric (fully printed) and according to patterns (dots with four diameters: 20 mm, 10 mm, 5 mm and 2 mm) (Figure 2). The percentage of the printed area, 19.63%, was the same in all four cases. After printing the samples were air dried and thermally cured.

Table 2. Printing, drying and curing conditions.

Phase	Conditions
Printing	Flat-screen stencil: mesh 43 threads/cm Printing speed: 80% Squeegee diameter: 8 mm Magnet pressure: level 5 No. of squeegee passes: 2
Drying	Air
Thermal curing	Ernst Benz TKF 15-M500 drier Temperature = 150 °C Time: 3 min

shows the conditions for printing, drying and curing.

2.3. Washing

Half of the printed (cured) specimens were washed in laboratory washing apparatus Launder-Ometer (SDL Atlas, Rock Hill, SC, USA) for 30 min at 40 °C according to the standard ISO 105-C06:2012 (E) [17] with a soap solution (4 g/L of standard detergent European Colorfastness Establishment (ECE)). After washing, the samples were air dried.

2.4. Fragrance Evaluation

Fragrance evaluation was performed for all unwashed and washed printed samples. For this test, five and 10 washings were performed. The method for fragrance evaluation was based on the Lewis method [18,19]. A part of the fabric was removed after the wash cycles (ISO 105-C06:2012 (E) standard), air dried for 24 h, and tested for the intensity of fragrance by five evaluators. The fragrance evaporation was stabilized by hanging the samples on a clothesline for 1 h. Afterward, the samples were taken to an evaluator in a room designated for evaluation. The evaluator scratched an “X” (about 3 cm × 3 cm) in the fabric with their fingernail to break some of the capsules, and then instantly smelled the sample. Finally, depending on the presence of the scent, he noted “strong,” “medium,”, “weak,” or “no” if the scent was not present. The evaluators conducted the test for 15 min maximum.

2.5. Fabric Properties

The properties of differently printed samples were examined. The fabric mass per unit area was determined according to SIST-EN 12127:1999 [20], the fabric thickness was measured according to the standard SIST EN ISO 5084:1999 [21], the fabric stiffness was evaluated with the ASTM D-1388-64 method A [22] and the fabric air permeability was determined according to SIST-EN ISO 9237:1999 [23].

2.6. Antibacterial Activity Testing

Since some of the essential oils have an antimicrobial activity effect, the antibacterial activity of the printed fabrics was evaluated for the Gram-negative bacteria Escherichia coli (ATCC 25923) and for the Gram-positive Staphylococcus aureus (ATCC 25922) according to SIST-EN ISO 20645:2005 [24]. Parts of fabrics (2 cm × 2 cm) were put on two-layer agar plates. The bottom layer consisted of a bacteria-free culture medium and the upper layer was inoculated with the selected bacteria. The level of antibacterial activity was evaluated by the size of bacterial growth in the contact zone between the agar and the sample and the width of the zone of inhibition around the sample. The tests were conducted in a certified laboratory. Prior to testing, some of the unwashed and washed samples were rubbed on a Martindale Pilling Tester. The rubbing was performed according to ISO/DIS 12945–2 [25] with 100 cycles without additional weight. The rubbing caused the microcapsules to open and release the active substance on the surface of the samples.

2.7. Resistance against Microorganisms in Soil

The resistance of printed cotton fabrics to the action of microorganisms in soil was determined. With the test, we wanted to check if the samples are biodegradable in soil or what effect different bacteria and fungi have on the fabric treated with the microcapsules under natural conditions. The test was performed according to the standards SIST EN ISO 11721-1: 2001 [26] and SIST EN ISO 11721-2: 2003 [27]. Textiles made of cellulose are resistant to the action of microorganisms in the soil if their structure, appearance and tensile strength have not changed significantly after burial. The samples were buried in humus soil containing 60 ± 5% of their maximum moisture capacity. The pH of the soil ranged from 4 to 7.5 (evident from the technical data of the soil) and it was not changed during the test. The strips 30 cm × 2 cm) of all samples were buried for five days. After a specified incubation period, the samples were dug up, rinsed with tap water, and immersed in 70% ethanol for 30 min. Then the specimens were air dried, and conditioned and tensile strength was measured according to the standard SIST ISO 5081: 1996 [28]. Resistance to deterioration was determined by measuring the relative reduction in tensile strength of the buried sample compared to the non-buried sample. After excavation of the samples, the changes in fabrics and microcapsules were analyzed using a scanning electron microscope (SEM).

2.8. Quantity of Formaldehyde

Formaldehyde is a poisonous gas, which is frequently applied to textiles in different processes. One of them is pigment printing, where formaldehyde arises from binders and crosslinkers. Formaldehyde can also arise from the melamine-formaldehyde wall of MC. Since we assumed that formaldehyde is present in the printed samples, we analyzed its presence. The amount of formaldehyde was measured photometrically according to Japan Law 112-1973 (acetylacetone method) [29]. This method determines both, the free formaldehyde on the fibers and that released during the extraction process.

3. Results and Discussion

3.1. Evaluation of Fragrance

A group of five evaluators assessed the fragrance on the unwashed and five and 10 times washed samples, respectively (

Table 3. Evaluation of fragrance.

Number of Washings	Sample	Number of Evaluations per Intensity of Fragrance
		Strong	Medium	Weak	No
Before washing	Fully printed	5	-	-	-
	20 mm dots	5	-	-	-
	10 mm dots	5	-	-	-
	2 mm dots	5	-	-	-
	Untreated	-	-	-	5
After 5 washings	Fully printed	-	5	-	-
	20 mm dots	-	5	-	-
	10 mm dots	-	5	-	-
	2 mm dots	-	5	-	-
	Untreated	-	-	-	5
After 10 washings	Fully printed	-	-	5	-
	20 mm dots	-	-	5	-
	10 mm dots	-	-	5	-
	2 mm dots	-	-	5	-
	Untreated	-	-	-	5

).

The ratings in Table 3 show that the scent is strongly present on all samples, regardless of the printed area. The concentration of microcapsules is sufficient for all samples to produce a strong fragrance on the fabric. After five washes, all samples emit a moderate scent and there is no difference between fully printed and patterned samples. The samples with dots smell the same. After 10 washes, the scent decreases but is still present on all printed samples. The microcapsules have impermeable walls that gradually open due to mechanical forces with each wash, so the fragrance is released and removed with the water. This is also evident from SEM micrographs in Figure 3, where printed unwashed, five times washed, and 10 times washed sample is presented. With more washings, more microcapsules were opened. From the results, it appears that after all washings, still some unopened microcapsules remain on the fabric and can release the fragrance.

3.2. Fabric Properties

The results of measuring mass per unit area, thickness, stiffness and air permeability of printed samples are shown in Figure 4, Figure 5, Figure 6 and Figure 7. The results are given graphically as the mean and standard deviation of five measurements for mass per unit area, thickness and air permeability, while for stiffness, 16 measurements were conducted.

Figure 4, Figure 5, Figure 6 and Figure 7 show that, as expected, the mechanical properties of fully printed fabrics differ from those of patterned fabrics.

A fully printed sample has a higher mass per unit area, is thicker, is stiffer and has lower air permeability than patterned samples. The biggest difference is seen in the results of measuring stiffness and air permeability. The fully printed sample is significantly stiffer and has much lower air permeability than all dot printed samples, where some areas of the fabric are not covered with microcapsules. Pattern printed fabrics are more comfortable, softer and airier than fully printed fabrics and are more suitable for various products.

The results of the washed samples differ from those of the unwashed ones. The mass per unit area of all washed samples increases after washing. We assume that this is a result of the shrinkage of the fabric after washing, which also leads to a decrease in the air permeability of the samples. The stiffness of all printed samples decreases with washing.

If you compare only the patterned fabrics, there is not much difference between them. The reason is the same percentage of printed area in all four cases.

3.3. Antibacterial Efficiency

The results of the antibacterial activity test showed that none of the samples exhibited antibacterial activity (similar to one of our previous articles [12]). All samples exhibited moderate bacterial growth and no zones of inhibition were present (Figure 8). This is apparently a consequence of the insufficient quantity of essential oils on the fabric.

3.4. Resistance to Microorganisms in Soil

To find out how the fabric coated with microcapsules of essential oils, formaldehyde and pigment system behaves when disposed of in nature, the soil test was conducted. Because of the high load of chemicals, only unwashed samples were tested. The relative reduction in tensile strength, q_red, of the buried sample compared to the unburied sample was calculated from the average tensile strength value of ten samples according to the equation:

$$q_{red,t}=\frac{F_t}{F_{t0}}$$

(1)

In which q_red is the reduction of tensile strength of the buried cotton specimen after the time of burial t, F_t is the tensile strength of the buried cotton specimen after the time of burial t and F_t0 is the tensile strength of the unburied cotton specimen. If the reduction in the tensile strength of the buried printed sample is less than 25% (q_red is greater than 0.75), it can be confirmed that the application on the fabric is resistant to rot. The results of measuring the tensile strength of the samples are shown in

Table 4. Tensile strength reduction of buried samples.

Sample	Ft of Unburied Sample (N)	Ft of Buried Sample (N)	qred of Buried Sample (%)
Untreated	307.59 ± 13.53	69.14 ± 0.90	77.52
Fully printed	299.81 ± 19.3	43.01 ±1.57	85.65
20 mm dots	307.29 ± 23.19	59.67 ± 0.73	80.58
10 mm dots	307.49 ± 14.43	58.90 ± 0.86	80.85
5 mm dots	306.45 ± 27.23	58.30 ± 1.10	80.98
2 mm dots	306.30 ± 22.73	58.55 ± 0.42	80.89

.

From the results in Table 4 (mean and standard deviation), it is evident that the sample with and without microcapsules are not resistant to decay. Actually, the printed samples (full or patterned) had worse resistance to breaking than the unprinted buried samples, as their reduction is higher than 80%. The fully printed sample had the worst breaking strength and is more decomposed than the untreated sample. From these results, we can conclude that despite many additives on cotton an indication of biodegradability was seen. The pigment system with microcapsules did not hinder the degradation of the printed samples.

SEM micrographs of unprinted and printed, unburied and buried, samples are shown in Figure 9.

Figure 9 shows that major morphological changes due to soil microflora functions occur in all buried samples, both untreated and printed. In the left column of the figure untreated sample is presented (Figure 9(1a–3a)) and in the right column the printed one is shown Figure 9(1b–3b). On the fibers of the untreated sample whose surface is smooth before burial (Figure 9(1a)), surface cracks and holes appear after 5 days of burial (Figure 9(2a,3a)). The fibers are obviously damaged and aren’t smooth anymore. On the samples printed with microcapsules (Figure 9(1b)) hyphae and mycelial entanglement occur during burial (Figure 9(2b)). The fibers are still covered with a pigment system. In some areas of the sample where microcapsules were visible before burial, only voids in the pigment system (polymer) are present after burial (Figure 9(3b)).

3.5. Free Formaldehyde Quantity

The amounts (mean results and standard deviation) of free formaldehyde from fully printed samples and samples printed according to patterns (in this analysis we used samples printed with smallest 2 mm dots and largest 20 mm dots), unwashed and washed are listed in

Table 5. Free formaldehyde quantity of fully printed samples and samples printed according to pattern.

Sample	Quantity of Formaldehyde (ppm)
	Unwashed	Washed
Without pattern	207.46 ± 4.3	39.29 ± 0.9
20 mm dots	75.92 ± 8.8	35.46 ± 0.6
2 mm dots	83.31 ± 7.2	37.84 ± 0.5

.

From the results in Table 5, the amount of formaldehyde in the fully printed samples is more than half that of the patterned samples, as expected. After washing, the amount of formaldehyde is much lower; there is not much difference between the differently printed samples. Oeko-Tex Standard 100 [30], demands that the level of free formaldehyde in the final product is lower than 75 ppm for products that are in skin contact, lower than 20 ppm for baby clothes and lower than 300 ppm for all other products. Therefore, all unwashed samples are not suitable for clothing. Patterned samples can be used for products that do not come into direct contact with skin, while fully printed samples can be used only for home textiles.

4. Conclusions

We can conclude that screen printing is a suitable method for applying fragrant microcapsules to cotton fabric. Printing them on the whole surface or in the form of patterns is appropriate. Thus, both methods are suitable for effective fragrance enhancement. The intensity of scent on all the samples similarly decreased with washing. The scent still remained after 10 washing cycles.

The mechanical properties of the fully printed fabric were different from the properties of the patterned fabric. The fully printed sample was stiffer and had lower air permeability than all the patterned samples. This was to be expected due to the larger coated area. There was not much difference between the dot printed samples due to the same percentage of the printed area. The pattern printed fabric is suitable for garment or home textile manufacturing as the comfort, softness and breathability of the fabric are in favor of the patterned samples.

None of the tested samples exhibited antibacterial activity. All samples exhibited moderate bacterial growth and no inhibition zones were present. All printed samples showed an indication of biodegradability as none of them were resistant to decay. All samples showed a reduction of more than 80% in their breaking strength after burial, and major morphological changes occurred in all samples. Therefore, we can conclude that the printed samples do not pose a threat to nature when disposed of in a landfill.

The amount of free formaldehyde on the fully printed fabric was more than half greater than the amount measured on the patterned fabric. After washing, the amount actually decreased and so did the difference between all samples. According to the Oeko-Tex Standard 100, which regulates its quantity in the finished textile, the patterned fabric can be worn without direct contact with the skin, and the fully printed fabric can be used for home textiles.

In summary, all the samples, both fully printed and patterned, are suitable for scented clothing not worn in direct contact with skin and for fabrics for technical use. Due to the better mechanical properties of the patterned samples and the lower amount of formaldehyde, printing in a pattern can be recommended.