Development of a novel curing system for low temperature curing of resins with the aid of nanotechnology and ultraviolet radiation

Abstract

Textile processing is an energy intensive process, which contributes about 15–20% of the cost of the finished product. The inefficient equipment and the non-optimized processes are the major causes of the energy losses. The most energy intensive process during wet processing is the curing. Conventionally curing/cross linking of resin is done at a high temperature of about 170 °C. In this research work, ZnO nanoparticles were used in the curing process with the aim to replace the conventional catalyst and to decrease the curing temperature and thermal curing time. The curing of resins was carried out using three different techniques i.e. thermal radiation, UV radiation and a combination of thermal & UV radiation. Promising results have been achieved.

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Introduction

Energy is one of the most important components in any industrial activity. The textile industry is one of the major energy consuming industries and retains a record of the lowest efficiency in energy utilization. In chemical wet processing the energy utilization rate is about 38% and thermal energy¹ is mostly used. Similarly, cotton fabric is one of the largest consumers of energy. Due to its high moisture regain it requires more heat to dry and cure as compare to other synthetic fabrics. But still it is the most commonly used fabric in the world due to its tremendous characteristics like comfort, absorbency, strength and much more. However, the problem of creasing in cotton fabric has perturbed its users for a long time.² The creasing phenomenon of cotton is associated with the presence of hydroxyl groups in the amorphous regions of cellulose. These week hydrogen bonds between the –OH groups can easily break and reform in the amorphous region when a moisture loaded cotton fabric is folded or pressed. This is due to the swelling of cotton fibres which causes the free movement of the internal polymer chains in amorphous regions, which tends the fibres to shrink, get creased.

To overcome this problem two approaches have been used by the researchers to reduce the swelling of cellulosic fabrics.³ One way is to close the pores of the cotton fibres by incorporating polymerized finish to prevent the entry of water molecules.⁴ This approach has the drawback as it clogs the fabric pores and deteriorate the breathability of the fabric, hence making it uncomfortable. The second and commercially used approach is the fixing of adjacent hydroxyl groups of the cellulose with multifunctional cross linkers called crease recovery finishes mostly resins which inhibit the swelling of fibres.

Resins are the bi-functional chemicals by nature, which form a crosslinking reaction with the hydroxyl groups of the cellulose and hinder the movement of cellulosic chains.^5,6 Dimethyloldihydroxyethyleneurea (DMDHEU) is widely used resin which is synthesized by the reaction of urea, glyoxal and formaldehyde. In DMDHEU the N-methylol groups create bonds with the –OH groups of the cellulose.^7,8 Maleic anhydride is another versatile chemical which is used to produce the lube oil additives, alkyl resins and unsaturated polyester resins.⁹

In resin finishing, catalyst is necessary to activate the crosslinking reaction and the most commonly used catalyst is MgCl₂, which is an acid liberating salt that allows the reaction to be carried out within 130–180 °C temperature range.¹⁰ MgCl₂ requires high temperature to crosslink the resin, hence requires high energy consumption. To combat this problem nanoparticles were used as catalyst for resin curing.

Nanoparticles of metals and metal oxides such as silver, zinc oxide & titanium dioxide have been used as antimicrobial agents for textiles with significant durability to washing.¹¹ The use of textile inorganic UV blockers in the form of nanoparticles e.g. ZnO and TiO₂ has also increased as compared to organic UV blockers because they are not toxic to environment and stable at high temperature and UV exposure. Moreover the nanoparticles offer greater surface area to volume ratio due to which their ability to block UV radiations is more than that of bulk materials and the effect produced by them is more durable. Other properties of the fabric which are affected by nano finishing are tensile strength, dye-ability, bursting strength, comfort, bending properties and frictional properties.^12,13

In this backdrop, the present research work was planned to use zinc oxide nanoparticles for curing of crease recovery finish to minimize the amount of catalyst (MgCl₂ and NaH₂PO₄) and to make the process economical. Moreover, the efforts will be made to replace thermal curing with UV curing, which is less costly by the use of ZnO nanoparticles.^14,15

Experimental

The experiments were carried out in followings phases.

Preparation of ZnO nanoparticles

ZnO nanoparticles were prepared by adding zinc acetate (1 g) and sodium hydroxide (5 g) each in 250 ml of methanol. After complete dissolution of both zinc acetate and sodium hydroxide in methanol, the sodium hydroxide solution was added drop wise to the zinc acetate solution through burette. This mixture was heated at 60 °C for approximately 2 hours, till the mixture became milky white, which indicates the appearance of nanoparticles.

Resin finishing of fabric samples

Two types of crease recovery finishes i.e. maleic anhydride and DMDHEU along with 50% of the weight of catalyst as NaH₂PO₄ and MgCl₂ were applied to 100% bleached cotton fabric. Treated samples were cured with and without nanoparticles using thermal, UV and combination of thermal & UV radiation. Samples were prepared by pad-dry-cure technique each with DMDHEU and maleic anhydride according to the research plan as given in Table 1. The samples dried at 120 °C for 2 minutes. After that six of the twelve samples were re-padded with ZnO and re-dried, then all the samples were cured using three different techniques. Thermal or conventional curing was done at 170 °C for 3 minutes. UV curing of the samples was done for 6 hours, (3 hour each side). In case of UV plus thermal cure samples were first UV cured for 2 hours (1 hour each side) and then thermally cured.

Table 1 Ultraviolet production of samples with DMDHEU resin

Resin type	Resin conc.	Catalyst	Curing technique
DMDHEU	5%	MgCl2	Thermal
			UV
	10%	MgCl2 + ZnO	Thermal & UV
Maleic anhydride	5%	NaH2PO4	Thermal
			UV
	10%	NaH2PO4 + ZnO	Thermal & UV

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The morphology of treated fabric was characterized by scanning electron microscope (HITACHI S-3500N). As samples were no conducting, therefore, they were metalized with ultrathin coating of gold in order to avoid the accumulation of static charges on surface and to obtain high resolution images. Crease recovery of the samples was tested on Shirley crease recovery tester using standard test method BS 3086-1972.¹⁶ The tear strength testing was performed using Elmendorf tear strength tester both in warp and weft direction, according to ASTM D 1424-09 (2013).¹⁷ A UV resistance test was also conducted using CAMSPEC 4550 UV visible spectrophotometer. UPF values of the samples were calculated according to AATCC 183. UPF stands for the term ultraviolet protection factor, the term that is used to express the amount of protection from ultraviolet rays provided to the wearer by the fabric. Another term used for this is SPF i.e. the Sun Protection Factor.¹⁸

Results and discussion

Characterization of nanoparticles

The Fig. 1 presents the morphology of treated cotton fibers. It can be seen that nanoparticles are deposited on fiber surface. They are present on all fibers as well as in interfiber spaces in the form of smaller and bigger aggregates.

Fig. 1 SEM images representing presence of ZnO nanoparticles on cotton fabric.

Crease recovery

The results of the crease recovery of the samples treated with DMDHEU are depicted in Fig. 2. It can be seen that the ZnO nanoparticles significantly increased the crease recovery angle of the fabric in both warp and weft direction without the use of any finish. This is attributed to the fact that the presence of nanoparticles inside the fibre structure restrict the movements of cellulosic chains in the amorphous region by filling the free spaces within the fibre structure.¹⁹

Fig. 2 Effect of ZnO nanoparticles on crease recovery.

Fig. 3 and 4 shows the results of crease recovery of fabric with and without ZnO particles with three different curing techniques. It can be seen that the best results are obtained using 10% resin concentration in combination with ZnO nanoparticles and MgCl₂.

Fig. 3 Crease recovery angle using DMDHEU resin.

Fig. 4 Crease recovery angle using maleic anhydride resin.

While comparing three different curing techniques, it can be seen that the best results are obtained by thermal curing in weft direction and by UV curing in warp direction. On UV activation, the electron present in the valence band jumps to conduction band of ZnO leaving a positive whole behind. This positive whole moves toward surface of nanoparticles and act a proton. It catalyzes the reaction between maleic anhydride and cellulose according to mechanism as proposed in Scheme 1. It has been reported in previous studies that photocatalysts enhanced the crease recovery angle of DMDHEU due to increased cross linking.²⁰

Scheme 1 Mechanism of reaction between maleic anhydride and cellulose in the presence of photoactivated ZnO.

On comparing the two resins it can be seen that DMDHEU showed better crease recovery angle than maleic anhydride. It is due to high reactivity of N-methylol groups of DMDHEU. These groups not only react with hydroxyl groups of cellulose but also undergo self-cross linking making a three dimensional network on fiber surface.² However, maleic anhydride does not undergo self-cross linking. It makes ester linkages only with the hydroxyl groups of cellulose. Due to extensive cross linking of DMDHEU, the fabric treated with it shows higher crease recovery angle.

Tear strength

The results of the samples corresponding to the tear strength of fabrics both in warp and weft wise direction are presented in Fig. 5. Considering the effect of ZnO nanoparticles, it is clear that the tear strength has decreased.

Fig. 5 Effect of ZnO particles on tear strength.

From Fig. 6, the same trend is visible with the increase in the resin concentration. This decrease can be attributed to the fact that during fabric tear, the yarns used to displace from their position and form yarn bunch against the tearing force. However, crosslinking with resin reduces the mobility of yarns thus reducing the bunching effect. Thus tear strength is reduced. However, this decrease can be compensated by decreasing the concentration of MgCl₂ and increasing ZnO nanoparticles.

Fig. 6 Effect on tear strength using DMDHEU.

While comparing the curing technique, the minimum tear strength value is resulted from thermal curing in weft and with UV curing in warp direction with 10% resin and ZnO nanoparticles.

The results of maleic anhydride on tear strength can be observed from Fig. 7. Lowest value of tear strength can be seen with 10% resin concentration. This decrease in tear strength value is attributed to the fact that the penetration of resin into interior of fabric reduces the internal plasticization of chains and imparts stiffness. Hence, chain slippage decreases due to which the tear strength decreases.

Fig. 7 Effect on tear strength using maleic anhydride.

UV protection factor

It can be seen from the Fig. 8, the UPF of the samples has significantly increased with the use of ZnO nanoparticles.

Fig. 8 Effect of ZnO nanoparticles on UV protection factor.

From Fig. 9 and 10 in both trials of samples treated with DMDHEU or maleic anhydride resin ZnO nanoparticles has increased the ultraviolet protection of fabric, but the effect is stronger with DMDHEU. The higher values of UPF is obtained at 5% concentration of finish with ZnO nanoparticles as catalyst by UV curing technique. On further increase in finish concentration impaired the UV blocking property of the fabric. Although the results obtained are not comparable with the internationally standard values marked for excellent UV protection fabric, these values confirmed the UV protection property of ZnO nanoparticles on cotton fabric.²¹ UPF can further be enhanced by process optimisation and controlling other factors required for ultimate UV shielding in fabric.

Fig. 9 Effect of nanoparticles on UV protection using DMDHEU resin.

Fig. 10 Effect of nanoparticles on UV protection using maleic anhydride.

Conclusions

As revealed from the above mentioned analysis, ZnO particles have successfully been used to cure the crease recovery finish under UV light and it can also be used in combination with other catalysts like magnesium chloride and sodium di hydrogen phosphate. It is suitable for curing of all types of crease recovery finishes like maleic anhydride and DMDHEU. The curing of resins under UV in the presence of ZnO nanoparticles can save significant amount of energy. Moreover with the use of ZnO nanoparticles a multifunctional property can be achieved like UV protection in addition to better crease recovery and tear strength properties.

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