Release behavior and kinetic evaluation of formaldehyde from cotton clothing fabrics finished with DMDHEU-based durable press agents in water and synthetic sweat solution

Abstract

The durable press-finished cotton fabrics were prepared with dimethylol dihydroxy ethylene urea (DMDHEU) and methylated DMDHEU, and tested for formaldehyde release in distilled water or synthetic sweat at pH 5 or 8 to simulate the extraction of formaldehyde by sweat under similar wearing conditions. The release mechanism of formaldehyde was studied and compared using three diffusion kinetic models. The effect of release conditions and finishing process on the formaldehyde release was also investigated. The results indicated that the release process of formaldehyde from the finished fabric includes a burst release phase and a slow release phase, which is explained by five different sources. The formaldehyde release mechanism followed the Fickian mode, and could be described by Higuchi, Double phases and Korsmeyer–Peppas kinetic equations, respectively. Higher formaldehyde release was found at pH 8 than that at pH 5. Formaldehyde showed a lower release rate in synthetic sweat solution than that in water under the same conditions. Increasing water volume or temperature enhanced the formaldehyde release, suggesting that a high sweat rate and skin temperature may accelerate the formaldehyde release. When finishing the fabrics, a higher concentration of finishing agent and the addition of softener accelerated the formaldehyde release. However, increasing catalyst concentration and curing temperature could reduce the formaldehyde release. It is concluded that formaldehyde release can be effectively reduced by optimizing the durable press finishing process of cotton fabrics, thus possibly decreasing risk of allergic contact dermatitis from formaldehyde in textiles.

Keywords

durable press finishing formaldehyde release dimethylol dihydroxy ethylene urea kinetic model

Durable press agents (DP agents) are generally used to improve the wrinkle resistance of various clothing fabrics during wear and laundering. However, the majority of DP agents often contain significant levels of formaldehyde because they are created from the condensation product of formaldehyde with urea or melamine. Formaldehyde is a known human carcinogen,^1,2 which is released from the cotton clothing fabrics finished with formaldehyde-based DP agents in the last decades. Previous study³ has found that large amounts of formaldehyde were concentrated in skin next to the durable press garment and relatively small amounts enter the body when rabbits were used as the test subjects. In the 1950s and 1960s, allergic contact dermatitis (ACD) from formaldehyde in textiles was commonly reported, especially in Europe. Repeated exposure to fabrics treated with N-methylol durable press agents served as the primary sensitizer in many of the cases.^4,5 The first sign of ACD is the presence of a rash or skin lesion at the site of exposure.

Since the 1960s dimethylol dihydroxy ethylene urea (DMDHEU) with lower levels of formaldehyde has gradually become a popular DP agent. In the last 30 years, modified DMDHEU (e.g. methylated or glycolated DMDHEU) was used to further reduce formaldehyde amounts. These values are well below the levels of 500–750 µgg⁻¹ estimated to be able to elicit ACD in patients who are allergic to formaldehyde.⁶ Additionally, the use of scavengers such as urea and carbohydrazide in durable press finishing systems of cotton fabrics has also been shown to be an effective way of reducing the formaldehyde release.⁷ Despite this, there are still several US clinical studies being reported^8–12 about US patients suffering from ACD to formaldehyde in textiles. Some countries such as Japan, Finland and China have legally limited the formaldehyde content in textile products to reduce the risk of ACD since 1973. The international lower limit (75–120 µgg⁻¹) for highly formaldehyde sensitive patients to develop dermatitis for the textiles in direct contact with the skin has also been proposed by de Groot and Maibach et al.^5,6

However, the threshold level for formaldehyde in clothes that may cause ACD in sensitized individuals is unknown. It is necessary to investigate the transfer mechanisms of formaldehyde from the finished fabric to human skin to ascertain how much formaldehyde is actually released to cause ACD. Hollies et al.¹³ reported that three mechanisms of transfer to skin require consideration: (1) direct transfer of solid material arising from contact between fabric and skin; (2) direct transfer of vapor released by the fabric under conditions of use; (3) direct transfer in the liquid phase of chemicals leached from the fabric by sweat. In the case of durable press-finished fabrics, it was found that the release of formaldehyde from cotton fabric through the first and second mechanism is very low (<10 ngg⁻¹).² Other researches^7,14,15 found that the formaldehydes released from the finished fabrics include free formaldehyde and hydrolysable formaldehyde from its precursors (the formaldehyde-containing compounds). If these precursors are present in the finished fabrics, they can be hydrolyzed by sweat on the skin surface to release free formaldehyde.

In practice, clothes, especially the underclothes, have frequent contact with skin and sweat. In general, two groups of people including workers involved in the production of finished textiles and consumers are considered to have the risk of formaldehyde contact allergy because both of them have close contact with the finished textiles. Thus, the third mechanism described above should be considered as the main route for the release of formaldehyde from the finished fabric to skin through sweat extraction. However, little is known about the transfer mechanism of formaldehyde molecules from the finished fabric to the human skin. Hence, the main objective of this work is to obtain the release rate and quantity of formaldehyde from the finished cotton fabrics with DMDHEU-based agents to simulate the extraction of formaldehyde by perspiration under similar wearing conditions. Moreover, the equilibrium and kinetic data of the release process in different aqueous media were evaluated and compared using three diffusion models (Higuchi, Double phases and Korsmeyer–Peppas equations) to study the release mechanism of the formaldehyde molecules from the finished fabric to the human skin. Some important variables including release condition (the ratio of the release a medium volume to mass of the finished fabric and temperature) and finishing process (DP agent concentration, catalyst concentration and curing temperature) on the formaldehyde release were investigated. In addition, this work also discussed the potential impact of ACD from formaldehyde release varied with sweat production on human skin.

Experimental methods

Durable press finishing of cotton fabrics

Commercially scoured, bleached and mercerized plain weave loom-state 100% cotton fabric with 32 yarncm⁻¹ density, 30 yarncm⁻¹ weft density and 112.3 gm⁻² average area density were used in this investigation, and further treated with an aqueous solution containing 2.0 gL⁻¹ Na₂CO₃ and 2.0 gL⁻¹ anionic detergent at 100℃ for 25min, and then thoroughly washed with cold distilled water and dried at room temperature. DMDHEU and methylated DMDHEU as two commercial DP agents were provided by China Textile Academy. Methylated DMDHEU was prepared by partial modification with methanol (equation (1)). These DP agents were chosen for study because they are widely used by the textile industry, especially in China. The contents of free formaldehyde of DMDHDU and methylated DMDHEU were determined to be 0.09% and 0.15%, respectively, by a method from the National Standard of China GB/T 27593-2011: Textile dyeing and finishing auxiliaries-free formaldehyde content of amino resin agent. A conventional pad-dry-cure process was adopted for the durable press finishing of the cotton fabric. It was impregnated with a finishing solution containing various concentration of DMDHEU or methylated DMDHEU (50–150 gL⁻¹) and MgCl₂6H₂O (0.5–20 gL⁻¹) with or without softener at room temperature. The impregnated fabric was then padded by a laboratory padding mangle (Mathis AG, Switzerland). The wet pick-up was 75–80%. After being padded, the fabric was immediately dried at 80℃ for 3 min and cured in an oven at a different temperature (140–190℃) for 1.5 min. All the finished fabrics were sealed using plastic bags in the dark at 5℃ before fabric evaluation.

Preparation of synthetic sweat solutions

The constituents of sweat on human skin mainly contain inorganic salts and organic compounds. Sweat pH is varied in its composition. The greater the sweat Na⁺ concentration, the greater the sweat pH.^16,17 The alkaline synthetic sweat (pH = 8.0) and acid synthetic sweat (pH = 5.0) were often used to simulate the actual sweat in some studies.^17–19 Two synthetic sweat solutions were prepared according to ISO standard test 105-E04 described in this work.^18,19 In order to produce the synthetic sweat solution, 0.50 gL⁻¹ L-histidine monochloride monohydrate, 5.0 gL⁻¹ NaCl and 2.20 gL⁻¹ NaH₂PO₄.12H₂O were mixed in distilled water. Subsequently, the pH value of this solution was adjusted to 8.0 using 0.10 molL⁻¹ NaOH for the alkaline synthetic sweat or 5.0 using 0.10 molL⁻¹ HCl for the acid synthetic sweat, and measured using a DHS-25C digital pH meter (Shanghai Jingmi Instrumental Co., China). All the solutions were stored in the dark at 5℃ before usage.

Release of formaldehyde from finished fabrics

The dried finished fabric was cut into small pieces (about 0.50 cm × 0.50 cm), and thoroughly mixed together. The pre-weighed fibrous pieces were placed into the fixed volume of the distilled water or synthetic sweat solution at atmospheric pressure (1.013 × 10⁵Pa) and a certain temperature under gentle shaking. The ratio of volume (ml) of release medium to mass (g) of the finished fabric (denoted as R_vm) was kept to be 100 (unless otherwise stated). After specified time interval, the fibrous pieces were removed by filtration. The concentration of the formaldehyde present in the aqueous medium at different time interval was then determined according to method of Japanese Law 112-1973 by adding acetylacetone at 40℃ for 30 min. The absorbance of the solution was measured at 412 nm using a UV-2401 Shimadzu spectrophotometer (Shimadzu Corp., Japan). All the experiments were carried out in triplicate. The results were expressed as the mean value. Standard deviation is less than 5% of the mean value.

Study of formaldehyde release kinetics

To investigate the mode of formaldehyde release from the finished fabrics, the release data were fitted to the following model equations:

Higuchi equation:^20–22

Q_{t} = k_{H} t^{0.5}

(2)

where Q_t is the concentration of formaldehyde released from the finished fabric at time t, and k_H represents the Higuchi release rate constant. Higuchi describes formaldehyde release from the finished fabric as a square root of the time-dependent process on the basis of Fickian diffusion.

Double phases equation:^23,24

Q_{t} = Q_{e} - ({Ae}^{k B t +} {Be}^{kSt})

(3)

where Q_e is the concentration of formaldehyde released from the fabric sample at equilibrium. Formaldehyde release can be divided into two phases: burst release phase (denoted as B phase) and slow release phase (denoted as S phase). k_B and k_S are the release rate constants for B and S phases, respectively.

Korsmeyer–Peppas equation:^20,25,26

Q_{t} = k_{KP} t^{n}

(4)

k_KP is the release rate constant and n is the release exponent, which is used to characterize the mechanism of the formaldehyde release and listed in Table 1.

Table 1.

Release mechanism with variation of n values

n value	Release mechanism	Rate as a function of time
n < 0.5	Quasi-Fickian diffusion	t ^0.5
n = 0.5	Fickian diffusion	t ^0.5
0.5 < n < 1.0	Anomalous (non-Fickian) diffusion	tⁿ ^-1
n = 1.0	Non-Fickian case II	Zero order release
n > 1.0	Non-Fickian super case II	tⁿ ^-1

Results and discussion

Formaldehyde release behavior

Cotton fabric was finished with DMDHDU or methylated DMDHEU by the pad-dry-cure process described above. Formaldehyde release from the obtained finished fabric was then carried out in different aqueous media at 35℃, and the experimental results are shown in Figure 1.

Figure 1.

Release curves of formaldehyde from two finished fabrics in different aqueous media. Finishing process: DMDHEU or methylated DMDHEU: 100 gL⁻¹, MgCl₂6H₂O: 18 gL⁻¹, Curing: 170℃ × 1.5 min. The values were given as the mean. Standard deviation is less than 5% of the mean value. DMDHEU: dimethylol dihydroxy ethylene urea.

Figure 1 shows that there is a sharp increase in release quantities of formaldehyde from two finished fabrics (Q_t) as time increasing at the initial release stage of 60 min. An insignificant increase of Q_t values was observed after 60 min, and the increasing tendency of Q_t values becomes level when release time is beyond 180 min. This is attributed mainly to several sources of released formaldehyde from the finished fabric. The first source is free formaldehyde adsorbed by the finished fabrics. This formaldehyde is generally defined as the uncombined monomeric formaldehyde present in finish solutions because its sources are stoichiometric excesses that may have been included in the resin manufacturing step.³ Free formaldehyde can quickly move from the fabric to water as soon as the fabric comes into contact with aqueous medium. The second source is the most labile cellulose hemiacetal, which readily revert back to formaldehyde (equation (5)) because cellulose often picks up formaldehyde from the atmosphere.²⁷ The third source may be formed by the hydrolysis of the N-CH₂OH groups from unreacted DMDHEU or from pendant N-CH₂OH groups of DMDHEU molecules that have reacted with cellulose through only one N-CH₂OH group. These linkages are rather labile and easily revert back to their starting materials (equation (6)). It is difficult to cure all the DMDHEU molecules which are applied and make sure there are no pendant N-methylol groups left. It is clear that this formaldehyde exhibits a relatively slower release rate than free formaldehyde since the hydrolysis of the N-CH₂OH groups depends on the relative stability of their C-O and C-N bonds.

The fourth source is the formaldehyde released from the breaking of the crosslinks between DMDHEU and cellulose molecules, which was revealed by the gradual disappearing of ether group (1108 cm⁻¹) as well as the formations of absorbing bands of -CH₂OH (1027 cm⁻¹ and 1077 cm⁻¹) for DMDHEU and absorption peak of -OH for cotton fiber in their IR spectra.^28,29 This will provide an additional N-CH₂OH group, which can hydrolyze, thus resulting in a slow formaldehyde release (equation (7)). It was reported that the methylated DMDHEU is more resistant to both acid and alkaline hydrolysis than those from DMDHEU.³⁰ As a result, methylated DMDHEU will give the fabrics low formaldehyde release. In addition, formaldehyde can be released directly from N, N′-oxydimethylene linkages formed by condensation of methylol groups of DMDHEU in the finished fabric (equation (8)).³¹ It should be noticed that the formaldehyde from pendant N-CH₂OH groups (third source), cellulose crosslinks (fourth source) and N, N′-oxydimethylene linkages is regarded as the hydrolysable formaldehyde, the released amounts of which are variable and larger than the amount of free formaldehyde.¹⁴ Accordingly, the release curves of formaldehyde can be divided into a burst release phase (denoted as B phase) and a slow release phase (denoted as S phase), as illustrated in Figure 1. The burst release phase may be explained by first source, second source, and partial hydrolysable formaldehyde. The rest of hydrolysable formaldehyde should be responsible for the slow release phase. A similar result was reported by Cooke.³

Mechanism of formaldehyde release

In order to understand the mechanism of formaldehyde release kinetics from the finished fabrics, the data presented in Figure 1 were regressed according to the three model equations, and the regression results are listed in Tables 2 and 3.

Table 2.

Kinetic parameters of formaldehyde release from DMDHEU finished fabric

Medium	pH	Models	Fitted model equations	Kinetic parameters	Correlation coefficient	MRE (%)	AIC
Water	5	Higuchi	Q_t = 10.40t^0.5 Q_t = 0.594t^0.5	k_HB = 10.40 k_HS = 0.594	R_HB = 0.9014 R_HS = 0.9728	7.648	193.7
		Double phases	Q_t = 96.52 − (42.06e^−0.699^t +54.28e^−0.063 ^t )	Q_e = 96.52	R = 0.9962	14.13	209.0
		Korsemeyer–Peppas	Q_t = 52.63t^0.117	k_KP = 52.63 n = 0.117	R_KP = 0.9134	18.3	125.2
	8	Higuchi	Q_t = 40.42t^0.5 Q_t = 1.491t^0.5	k_HB = 40.42 k_HS = 1.491	R_HB = 0.9866 R_HS = 0.9651	35.74	176.9
		Double phases	Q_t = 389.1 − (288.6e^−0.027 ^t +94.32e^−29.86 ^t )	Q_e = 389.1	R = 0.9988	−72.27	185.8
		Korsemeyer–Peppas	Q_t = 121.1t^0.211	k_KP = 121.1 n = 0.211	R_KP = 0.9505	−32.96	264.5
Synthetic sweat	5	Higuchi	Q_t = 8.61t^0.5 Q_t = 0.859t^0.5	k_HB = 8.61 k_HS = 0.859	R_HB = 0.9950 R_HS = 0.9693	−17.57	105.4
		Double phases	Q_t = 79.66 − (20.07e^−0.007 ^t +53.51e^−0.052 ^t )	Q_e = 79.66	R = 0.9962	26.43	213.6
		Korsemeyer–Peppas	Q_t = 20.29t^0.240	k_KP = 20.29 n = 0.240	R_KP = 0.9547	−4.53	125.0
	8	Higuchi	Q_t = 29.12t^0.5 Q_t = 3.413t^0.5	k_HB = 29.12 k_HS = 3.413	R_HB = 0.9721 R_HS = 0.9790	10.64	202.5
		Double phases	Q_t = 282.4 − (105.8e^−0.012 ^t +152.5e^−0.110 ^t )	Q_e = 282.4	R = 0.9926	−43.66	182.8
		Korsemeyer–Peppas	Q_t = 92.51t^0.200	k_KP = 92.51 n = 0.200	R_KP = 0.9635	−25.48	236.4

k_HB (min^−0.5) and k_HS (min^−0.5) are Higuchi release rate constants at B phase and S phase, respectively. k_KP (min⁻ⁿ) is the release rate constant for Korsemeyer–Peppas model. Q_e (μgg⁻¹) is the concentration of formaldehyde released from the fabric sample at equilibrium. The values were given as the mean. Standard deviation is less than 5% of the mean value. MRE (maximum relative error) is express as: MRE = (1-Q_fit/Q_exp) × 100%, where Q_ex and Q_fit are experimental data and fitted value of formaldehyde release quantity. AIC (akaike information criterion) can be calculated as: AIC = N(ln SSR) + 2p, where N is number of experimental data points, SSR is the sum of the squared residuals and p is the number of parameters. The smaller the value of AIC, the better the model adjusts the data.³²

Table 3.

Kinetic parameters of formaldehyde release from methylated DMDHEU finished fabric

Medium	pH	Models	Fitted model equations	Kinetic parameters	Correlation coefficients	MRE (%)	AIC
Water	5	Higuchi	Q_t = 7.801t^0.5 Q_t = 1.829t^0.5	k_HB = 7.801 k_HS = 1.829	R_HB = 0.9613 R_HS = 0.9522	18.76	180.4
		Double phases	Q_t = 90.82 − (42.04e^−0.014 ^t +48.40e^−0.773 ^t )	Q_e = 90.82	R = 0.9845	13.01	216.6
		Korsemeyer–Peppas	Q_t = 38.46t^0.151	k_KP = 38.46 n = 0.151	R_KP = 0.9866	−16.87	156.0
	8	Higuchi	Q_t = 22.13t^0.5 Q_t = 2.706t^0.5	k_HB = 22.13 k_HS = 2.706	R_HB = 0.9456 R_HS = 0.9354	31.61	182.8
		Double phases	Q_t = 223.7 − (79.79e^−0.011 ^t +129.56e^−0.156 ^t )	Q_e = 223.7	R = 0.9939	−13.94	138.3
		Korsemeyer–Peppas	Q_t = 84.71t^0.171	k_KP = 84.71 n = 0.171	R_KP = 0.9643	−29.53	217.7
Synthetic sweat	5	Higuchi	Q_t = 4.29t^0.5 Q_t = 0.388t^0.5	k_HB = 4.29 k_HS = 0.388	R_HB = 0.9881 R_HS = 0.9522	15.24	128.2
		Double phases	Q_t = 47.84 − (31.65e^−0.021 ^t +16.22e^−0.614 ^t )	Q_e = 47.84	R = 0.9969	−38.42	203.5
		Korsemeyer–Peppas	Q_t = 15.89t ^0.198	k_KP = 15.89 n = 0.198	R_KP = 0.9540	−12.40	84.03
	8	Higuchi	Q_t = 12.55t^0.5 Q_t = 0.801t^0.5	k_HB = 12.55 k_HS = 0.801	R_HB = 0.9419 R_HS = 0.9135	−7.753	95.17
		Double phases	Q_t = 114.2 − (39.81e^−0.018 ^t +75.61e^−0.169 ^t )	Q_e = 114.2	R = 0.9991	−32.74	140.7
		Korsemeyer–Peppas	Q_t = 47.56t^0.117	k_KP = 47.56 n = 0.117	R_KP = 0.9156	−96.45	233.9

As shown in Tables 2 and 3, the data are found to fit well with the Higuchi equation with all regression coefficients >0.90, illustrating that the formaldehyde release from the finished fabrics can be described using the Higuchi equation based on Fickian diffusion by two stages. This is because the Higuchi diffusion-controlled release model is suitable for the highly crosslinked polymer network, which makes the polymeric segments more rigid and restricts their relaxation upon contacting with incoming solvent molecules.^21,22 The DMDHEU-based DP agents can react with hydroxyl groups of cellulose chains to produce the crosslink structures on the surface of the finished fabrics. On the other hand, the formaldehyde release can be best expressed by Double phases model, as the fitting results showed a higher regression coefficients (>0.98). This indicates that the formaldehyde release has been composed of a burst release phase and slow release phase. This is in good agreement with the result obtained according to Higuchi equation. To further confirm the release mechanism, the data were also fitted into Korsmeyer–Peppas equation for calculating the diffusion exponent (n). Two finished fabrics showed better regression coefficients (>0.90), with slope (n) values ranging from 0.10 to 0.30, indicating that the formaldehyde release follows Fickian mode.^20,25,26 Besides, MRE and AIC values of these fitted model equations were in their reasonable ranges. According to Fick’s first law, the flux goes from regions of high concentration to regions of low concentration, with a magnitude that is proportional to the concentration gradient. Consequently, it is believed that formaldehyde will move from a region of high concentration on the fabric to a region of low concentration in the aqueous solution across a concentration gradient. Formaldehyde release from the DMDHEU or methylated DMDHEU finished fabrics has the same mechanism at pH 5 and 8 in water or synthetic sweat solution. Moreover, it is also noticed that k_HB is much higher than k_HS for each of the formaldehyde release curves for two DP agents in water or synthetic sweat solution. This confirms that formaldehyde has a significant burst release phase once the finished fabric is immersed with water or synthetic sweat solution.

In addition, it is evident in Tables 2 and 3 that k_HB, k_KP and Q_e values at pH 5 are lower than those at pH 8 for two finished fabrics in water or synthetic sweat solution, which demonstrates that formaldehyde release has more difficulty at acid medium than at alkaline medium under the same conditions. This is mainly owing to the relative stability of the C-O and C-N bonds of the N-CH₂OH in DMDHEU or methylated DMDUEU and its crosslinks with cellulose molecules. It was reported that the rate of cleavage of the C-O bond is much faster than that of the C-N bond under acidic conditions, thus the latter determines the release rate of formaldehyde. Moreover, the rate of cleavage of the C-N bond was relatively lower under acid condition and higher under alkaline condition,³ which therefore enhanced the formaldehyde amount released from the finished fabric at alkaline medium. The cleavage of C-O bond and C-N bond of the N-CH₂OR under acidic condition is expressed in equation (9).

On the other hand, formaldehyde release shows larger k_HB, k_KP and Q_e values in the case of water than in the case of synthetic sweats at the same pH level. The main reason may be that the NH or NH₂ groups in L-histidine molecule can react with formaldehyde,^33,34 and their reaction is described by equation (10).

In order to verify the reaction of formaldehyde with L-histidine in synthetic sweat, each component of synthetic sweat was added into three aqueous formaldehyde solution (initial concentration: 5.0 mgL⁻¹), respectively. And then the resulting mixtures were kept at pH 5 or pH 8 and 30℃ for 30 min under continuous agitation. For comparison, a control experiment was also performed with synthetic sweat. Finally, the formaldehyde concentrations in four solutions were measured and are presented in Table 4.

Table 4.

Formaldehyde concentrations in different solutions after reaction

Systems	Formaldehyde concentrations (mgl⁻¹)
Systems	pH = 5	pH = 8
5.0 gl⁻¹ NaCl	4.99	4.92
2.2 gl⁻¹NaH₂PO4.12H₂O	4.97	4.95
0.50 gl⁻¹ L-histidine	3.50	3.62
Synthetic sweat	3.54	3.57

Table 4 shows that the formaldehyde concentration in the solution containing L-histidine or synthetic sweat is much lower than 5.0 gl⁻¹. By contrast, the formaldehyde concentrations in the other two systems without L-histidine are close to 5.0 gl⁻¹ under the same conditions. These results confirm that L-histidine has reacted with formaldehyde, thus reducing the formaldehyde concentration in the solution or synthetic sweat synthetic sweat.

Comparing the kinetic parameters of formaldehyde release listed in Tables 2 and 3, k_HB, k_KP and Q_e values for DMDHEU-finished fabric are remarkably higher than those for methylated DMDHEU-finished fabrics. This is because that the alkylation of DMDHEU can significantly reduce the formaldehyde release by improving the stability of C-O bond of its N-CH₂OR group during the hydrolysis. Moreover, the resistance to hydrolysis also seems to depend on the extent of alkylation of DMDHEU.³⁰ Another possible reason is that DMDHEU has a higher content of free formaldehyde than methylated DMDHEU, as described above. Thus, more free formaldehyde molecules may be transported to the fabric when DP finishing being conducted with DMDHEU. Hence, it is believed that there is higher formaldehyde concentration on DMDHEU-finished fabric than that on methylated DMDHEU-finished fabric. On the basis of Fick diffusion law, formaldehyde molecules move more quickly from DMDHEU-finished fabric than methylated DMDHEU-finished fabric into the aqueous solution. More importantly, it should be pointed out that Q_e values (79.66 µgg⁻¹ for DMDHEU and 47.84 µgg⁻¹ for methylated DMDHEU) in acidic synthetic sweat for two finished fabrics are lower than the international lower formaldehyde limit (75–120 µgg⁻¹) for the textiles in direct contact with the skin.^5,6 Correspondingly, Q_e values (282.4 µgg⁻¹ for DMDHEU and 114.2 µgg⁻¹ for methylated DMDHEU) in alkaline synthetic sweat significantly exceed this limit. Accordingly, from the view point of reducing formaldehyde release, it is necessary to decrease the application of two DP agents, especially DMDHEU in wrinkle-resistant finishing of cotton fabrics.

Effect of release conditions

It is well known that the amount of sweat secreted from the pores of the human body or the sweat rate is highly varied and dependent upon daily activity, emotional state and environmental temperature. Moreover, human sweat production may increase to 3.0 Lh⁻¹ by short-term exercise in warm environment.³⁵ To examine the formaldehyde release from the finished fabric in different amount of sweat secreted, cotton fabric was finished with DMDHEU or methylated DMDHEU using the pad-dry-cure process, and then tested for formaldehyde release in distilled water at 35℃ and different R_vm for 400 min. The released quantity of formaldehyde at equilibrium (Q_e) was calculated by Double phases equation and provided in Figure 2.

Figure 2.

Relationship between R_vm and Q_e values. Finishing process: DMDHEU or methylated DMDHEU: 100 gL⁻¹, MgCl₂6H₂O: 18 gL⁻¹, Curing: 170℃ × 1.5 min. Error bars are the standard deviation of the mean values shown. DMDHEU: dimethylol dihydroxy ethylene urea.

Figure 2 shows that little change in Q_e value at pH 5 was observed when R_vm level increased from 10 to 100. This result proposes that in practice, formaldehyde released is hardly affected by increasing the amount of sweat on the surface of human skin. In contrast, increasing R_vm level is accompanied by proportionally increasing Q_e value at pH 8. Furthermore, Q_e values with higher R_vm levels (R_vm > 10 for DMDHEU or R_vm > 50 for methylated DMDHEU) are higher than the international lower formaldehyde limit (75–120 µgg⁻¹) for the textiles in direct contact with the skin. This indicates that higher sweat amount may enhance the formaldehyde release from the fabric when sweat pH becomes neutral or alkaline with the increased sweat rate, which may increase the risk of formaldehyde contact allergy, due to the fact that sweat of low sweat rates is acid, whereas that of high sweat rates is basic.³⁶ Besides, it is well recognized that sweat rate and pH are not uniform over the skin surface. The sweat rate and pH at chest, scapula and lower back are significantly higher than those at other skin regions, especially abdomen and upper arm.¹⁶ Hence, it is believed that these skin regions may easily cause formaldehyde contact allergy at the same conditions. In addition, DMDHEU-finished fabric exhibits much higher Q_e value than methylated DMDHEU-finished fabric at the same R_vm level.

Sweat temperature is greatly determined by skin temperature. The mean skin temperatures have been tested to be between 33–38℃.³⁷ To evaluate the effect of sweat temperature on formaldehyde release, two finished fabrics described above were employed for releasing formaldehyde into distilled water at different temperatures for 400 min, and the results are shown in Table 5.

Table 5.

Effect of temperature on k_HB, k_HS and Q_e values at pH 5 and 8

Temperature (℃)		DMDHEU				Methylated DMDHEU
Temperature (℃)		30	33	36	42	50	75	100	125
k_HB (min^−0.5)	pH = 5	8.190	9.022	10.18	11.19	3.123	3.651	3.988	4.662
k_HB (min^−0.5)	pH = 8	21.50	24.73	28.41	32.36	10.19	12.89	14.45	16.03
k_HS (min^−0.5)	pH = 5	1.591	1.521	1.047	1.244	1.649	1.799	1.914	2.001
k_HS (min^−0.5)	pH = 8	4.707	5.699	5.710	5.789	1.725	2.422	2.787	2.890
Q_e(µgg⁻¹)	pH = 5	86.04	94.45	100.8	105.9	29.01	33.57	39.35	51.64
Q_e(µgg⁻¹)	pH = 8	228.0	260.9	296.5	324.4	98.73	121.9	138.46	153.8

Finishing process: DMDHEU or Methylated DMDHEU: 100 gL⁻¹, MgCl26H2O: 18 gL⁻¹, Curing: 170℃ × 1.5 min. Error bars are the standard deviation of the mean values shown. The values were given as the mean. Standard deviation is less than 5% of the mean value.

Table 5 shows that k_HB, k_HS and Q_e values at pH 8 gradually increase with rising temperature from 30℃ to 42℃, whereas the three values at pH 5 vary insignificantly with temperature. These results demonstrate that raising temperature can significantly accelerate formaldehyde liberating from the fabric at burst release phase. The reason may be that the release rate of free formaldehyde and the hydrolysis of pendant N-CH₂OR are promoted by rising temperature.³ Thus, it is predicted that relatively higher skin temperature may be beneficial for the transfer of formaldehyde from fabric to skin surface, thus leading to formaldehyde contact allergy more easily.

Effect of finishing process

DP agent concentration

DP agent can crosslink the adjacent cellulose chains for improving wrinkle resistance of cotton fabric. In this work, cotton fabrics were finished with various amounts of DMDHEU or methylated DMDHEU and 18 gL⁻¹ MgCl₂6H₂O by curing treatment (170℃ × 1.5 min), and the formaldehyde release from the obtained finishing fabrics was conducted in distilled water at 35℃. Their k_HB and k_HS values were then determined through the method mentioned above and shown in Table 6.

Table 6.

Effect of DP agent concentration on k_HB, k_HS and Q_e values

DP agents		DMDHEU (gL⁻¹)					Methylated DMDHEU (gL⁻¹)
DP agents		50	75	100	125	150	50	75	100	125	150
k_HB (min^−0.5)	pH = 5	5.337	5.813	6.808	8.547	9.993	1.290	2.872	4.425	5.987	6.083
k_HB (min^−0.5)	pH = 8	17.95	26.16	41.19	63.75	93.88	9.333	12.09	16.11	20.09	24.92
k_HS (min^−0.5)	pH = 5	0.335	0.536	0.690	1.030	1.311	1.234	1.627	1.923	2.298	2.661
k_HS (min^−0.5)	pH = 8	3.635	4.147	4.891	6.557	9.380	0.871	1.890	2.706	3.987	5.655
Q_e(µgg⁻¹)	pH = 5	46.65	56.55	68.75	85.00	104.2	11.35	30.64	48.45	60.95	72.07
Q_e(µgg⁻¹)	pH = 8	192.8	261.1	406.6	664.1	908.6	103.97	125.36	167.46	215.57	232.57

The values were given as the mean. Standard deviation is less than 5% of the mean value.

Table 6 shows that k_HB, k_HS and Q_e values, especially at pH 8, gradually increase as concentration of DP agent increases from 50 gL⁻¹ to 150 gL⁻¹, suggesting that higher concentration of DP agent in the padding bath gives rise to a rapid formaldehyde release from the finished fabric. This is mainly due to the increasing amount of free formaldehyde and hydrolysable formaldehyde from the high content of the unreacted DP agent on the finished fabric when larger dosages of DP agent being applied. The resulting pendant N-CH₂OR groups can hydrolyze to produce formaldehyde.

Catalyst concentration

MgCl₂ is the most common Lewis acid catalyst usable for durable press finishing. In this experiment, the finished fabrics were prepared with various amounts of MgCl₂6H₂O and 100 gL⁻¹ DMDHEU or methylated DMDHEU by curing treatment (170℃ × 1.5 min), and tested for formaldehyde release in distilled water at 35℃. Their k_HB, k_HS and Q_e values were determined and are presented in Table 7.

Table 7.

Effect of catalyst concentration on k_HB, k_HS and Q_e values

Catalyst (gL⁻¹)		DMDHEU					Methylated DMDHEU
Catalyst (gL⁻¹)		0.5	5	10	15	20	0.5	5	10	15	20
k_HB (min^−0.5)	pH = 5	18.73	15.62	13.27	10.96	3.721	19.04	14.75	13.12	6.026	20.50
k_HB (min^−0.5)	pH = 8	122.5	71.73	55.62	35.02	14.63	47.22	35.83	25.94	21.09	13.5
k_HS (min^−0.5)	pH = 5	3.752	2.903	2.736	2.382	0.7126	3.893	2.942	2.323	1.059	3.842
k_HS (min^−0.5)	pH = 8	21.058	11.704	10.48	7.367	2.363	8.081	6.090	4.988	4.456	2.896
Q_e(µgg⁻¹)	pH = 5	194.0	144.1	138.9	98.69	31.38	179.20	150.2	120.3	57.88	19.08
Q_e(µgg⁻¹)	pH = 8	1254	736.8	547.1	340.8	133.7	459.1	345.8	263.2	210.2	128.4

The values were given as the mean. Standard deviation is less than 5% of the mean value.

Table 7 shows that a higher concentration of catalyst causes the significant decrease in k_HB, k_HS and Q_e values, particularly at alkaline medium. Moreover, a Q_e value lower than 150 µgg⁻¹ was found when 20 gL⁻¹ MgCl₂6H₂O was used, since it can lead to more intensive linkages of DP agent with cellulose fibers to reduce the unreacted DP agent and pendant N-methylol groups, thus inhibiting the formation of hydrolysable formaldehyde.

Curing temperature

Cotton fabrics padded with 100 gL⁻¹ DMDHDU or methylated DMDHEU and 18 gL⁻¹MgCl₂6H₂O were cured at different temperatures for 1.5 min. Formaldehyde release of the finished fabric in distilled water was then performed and their k_HB, k_HS and Q_e values are given in Table 8.

Table 8.

Effect of curing temperature on k_HB, k_HS and Q_e values

Curing temperature (℃)		DMDHEU				Methylated DMDHEU
Curing temperature (℃)		130	150	170	190	130	150	170	190
k_HB (min^−0.5)	pH = 5	18.46	13.63	5.155	3.608	8.422	4.621	3.853	2.721
	pH = 8	58.47	31.63	12.63	8.379	20.46	16.72	6.072	4.836
k_HS (min^−0.5)	pH = 5	3.950	2.281	0.991	0.754	1.519	0.789	0.731	0.553
	pH = 8	13.74	7.471	2.829	1.811	3.835	3.915	1.208	0.941
Q_e(µgg⁻¹)	pH = 5	178.3	129.3	56.09	39.28	87.81	42.11	32.87	21.29
	pH = 8	579.2	300.1	110.2	79.21	199.2	167.7	67.34	47.22

The values were given as the mean. Standard deviation is less than 5% of the mean value.

Table 8 shows that k_HB, k_HS and Q_e values of the finished fabrics decreased with elevation of curing temperature. This result demonstrates that a higher curing temperature is able to enhance the degree of linkage between DP agent and cotton fiber, therefore resulting in a reduced release of hydrolysable formaldehyde. However, the free formaldehyde is the most independent upon curing temperature.

Softeners

Use of a softener in the durable press formulation partially overcomes the loss of strength, stiffness and resistance to abrasion of the finished fabric owing to crosslinking of DP agent.³⁸ In this work, three commercial softeners including silicone softener AMS, polyurethane softener DPU and polyethylene softener PEN were selected to prepare three different DMDHEU-finished cotton fabrics. Meanwhile, the finished cotton fabric was also prepared as a control sample without softener using the same method. They were then investigated for formaldehyde release. The contact angle of a water droplet on the finished fabric surface was measured using a liquid-solid contact angle analyzer (DSA100; Krüss, Germany) equipped with a high-speed camera and calculated by Tangent method 2. The experimental results were presented in Table 9.

Table 9.

k_HB, k_HS and Q_e values and water contact angles of the finished fabrics with different softeners

DP agents Softeners		DMDUEU				Methylated DMDHEU
DP agents Softeners		Control	AMS	DPU	PEN	Control	AMS	DPU	PEN
k_HB (min⁻¹)	pH = 5	7.673	9.393	11.98	14.71	4.532	6.342	8.391	10.02
k_HB (min⁻¹)	pH = 8	22.50	50.18	58.04	76.55	15.98	20.39	27.01	30.63
k_HS (min⁻¹)	pH = 5	1.898	2.048	3.099	3.238	1.423	1.966	2.766	2.904
k_HS (min⁻¹)	pH = 8	6.503	7.587	8.903	11.38	2.391	2.502	3.541	4.223
Q_e(µgg⁻¹)	pH = 5	75.48	95.35	111.1	135.1	42.88	69.54	80.37	101.5
Q_e(µgg⁻¹)	pH = 8	217.0	496.3	573.8	771.8	159.74	198.43	274.8	298.51
Water contact angle (^O)		10.3	135.1	47.3	37.5	10.1	142.8	51.2	41.6

Finishing process: DP agent: 100 gL⁻¹, MgCl₂6H₂O: 18 gL⁻¹, softener: 30 gL⁻¹, Curing: 170℃ × 1.5 min. The values were given as the mean. Standard deviation is less than 5% of the mean value.

It can be seen from Table 9 that k_HB, k_HS and Q_e values of the finished fabrics with softener are higher than those of control sample at the same conditions, indicating that the addition of softener can enhance the release rate of formaldehyde from the fabrics. This is because formation of the softener film on the fabric surface during cure treatment may block the crosslink of DP agent with cotton fiber to increase the amount of the unreacted DMDHEU or methylated DMDUEU and pendant N-methylol groups, which can generate the hydrolysable formaldehyde. On the other hand, it is worth noticing that the film formed with softeners, particular in silicone softener AMS can significantly improve the surface hydrophilicity of the finished cotton fabric. As seen in Table 9, according to their water contact angle, four samples were ranked in this order: AMS > DPU > PEN > Control. This indicated that the hydrophilicity is the lowest for Control, follow by DPU and PEN, and the highest for AMS. As a result of increasing hydrophilicity, the water wetting and diffusion on the finished fabric are hindered; hence this limits the production of hydrolysable formaldehyde and free formaldehyde moving from the finished fabric to water. Specially, the water contact angle of the finished fabric with AMS is 135.3–142.8^o, much higher than those of the finished fabric with the other softeners. Thus, it has a stronger limiting effect on formaldehyde release than the other softeners under the same conditions. The reason is that the Si-H group of silicon polymer hydrolyzes to the silanol and condenses to a three-dimensional resinous film, making the fabric highly water repellent.³⁸ Consequently, the presence of softener in the durable press formulation obtains better performance of the finished fabric, but promotes formaldehyde release at the same time.

Conclusions

The release process of formaldehyde from the finished cotton fabrics with DMDHEU-based DP agents included two stages, namely the burst release phase and the slow release phase, which is explained by five different sources. Formaldehyde has a higher release rate at the burst phase than the slow phase from the fabrics under the same conditions. More importantly, the formaldehyde release mechanism followed Fickian mode, and described using three different kinetic equations including Higuchi, Double phases and Korsmeyer–Peppas models, respectively. Higher formaldehyde release was found in alkaline medium than in acid medium. Formaldehyde showed a lower release rate in synthetic sweat solution than that in water under the same conditions due possibly to the reaction of L-histidine with formaldehyde. Although the methylated DMDHEU finished fabric has the similar formaldehyde release mechanism to the DMDHEU finished fabric, its release rate and quantity at equilibrium are much less than those of the latter under the same condition. The formaldehyde release quantity of two finished fabrics in alkaline synthetic sweat exceeds the lower limit for causing the formaldehyde contact allergy, therefore the two DP agents, especially DMDHEU should be limited for the treatment of cotton fabric. Increasing water volume or temperature enhanced the formaldehyde release, suggesting that high sweat rate and skin temperature may accelerate the formaldehyde release, thus increasing the risk of ACD from formaldehyde in textiles. Furthermore, formaldehyde release is significantly affected by DP finishing process. Higher concentration of DP agent and the addition of softener enhance the formaldehyde release. On the contrary, increasing catalyst concentration and curing temperature can reduce the formaldehyde release. Therefore, the formaldehyde release can be effectively reduced by optimizing the DP finishing process of cotton fabrics, possibly eliciting ACD in patients allergic to formaldehyde under real wearing conditions.

Footnotes

Acknowledgement

The authors thank the Tianjin Municipal Science and Technology Committee for a Research Program of Application Foundation and Advanced Technology (11JCZDJC24600).

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 work was supported by the Natural Science Foundation of China (grant number 20773093).

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