2. New technologies for textile functionalization

2.5. Microencapsulation process

Microencapsulation - general

A wide range of functional finishing of fabrics can be obtained by using a microencapsulation technique , which is widely used in the pharmaceutical industries and belong nanotechnology but here will be presented and explained as separate subchapter. 

Beside a quite number of PCMs, for textile applications is limited on few of them, since the PCMs in textiles are required to have suitable temperature range, high latent heat capacity, a stable chemical property and be non-toxic and comfortable. According to the mentioned  requirements, paraffin was found to be one of the most promising PCMs for textiles. To avoid the leakage of PCMs while transferring to the liquid state, encapsulation of PCMs was developed and has become the most effective method.

This technology enables to carry out several liquid or solid agents (fragrant, flame-retardant agents, antibacterial, etc.) that are encapsulated in phase-changing materials acting as binders (e.g. wax) and can be used to develop odor-eliminating finishes of fabrics, as well as fire-retardant and anti-microbial agents that can also be microencapsulated for advanced fabric finishing. 

Classification of phase change materials (PCMs)

                                                                                     Usuful link/video: 


Substances for microencapsulation

  1. PCM (Phase change materials) definition (usuful link: https://en.wikipedia.org/wiki/Phase-change_material)

Phase change materials (PCMs) are capable of absorbing or releasing a large amount of energy in the form of latent heat at a narrow temperature range during phase transitions, which could be solid–solid, solid–liquid,solid–gas, liquid–gas or vice versa. PCMs with solid–liquid phase transition are most widely used due to their high heat storage capacity, limited volume change and wide range of temperature adjustment.

      

Core materials:  PCM microcapsules are made from paraffin, organic non-paraffins (fatty acids), salt hydrates, low melting point metals and alloys, or commercial PCMs,

Shell materials: Polymethyl methacrylate (PMMA), polystyrene or polyurethane, etc.

Microencapsulation techniques can be classified into three major categories for functional textiles:
        • chemical processes (interfacial polymerization, in-situ polymerization, photopolymerization),
        • physico-chemical processes  (simple and complex coacervation and molecular inclusion),
        • physical processes (spray drying, solvent evaporation).

Application of PCMs for:
  • thermo-recycling systems, 
  • energy saving building materials, 
  • smart materials, 
  • astronaut suits against the extreme temperature fluctuations in outer space, 
  • thermoregulating textiles.

 Methods of application of PCM microcapsules

A) During the fiber spinning - wet and dry process from solution, melt, electrospinning.

     Wet spinning of fiber
     1. Synthesis of the spinning solution
The spinning solution, including sodium alginate, paraffin wax, emulsifier and water, was prepared at a defined concentration. 
     2. Wet spinning process
By using a customized industrial wet-spinning device showed at figure below, an continuous alginate fibers could be obtained. 
  • spinning microemulsion was extruded from a commercial spinneret by providing 0.5 MPa nitrogen gas, which controlled by a pressure regulator,
  • spinneret contained 50 holes, each diameter of 70 μm.,
  • extruded microemulsion went through a 4% CaCl2 coagulation solution at a temperature of 35ºC for 3 minutes,
  • spun alginate fibers had a draft ratio of 1.5, and were dried at room temperature.
Finally, the calcium alginate fiber containing PCM was formed for evaluation according to schematic approach in Figure below. 

          

            



       SEM morphology of calcium alginate fiber containing spherical PCM 



       Processes of phase change material (PCM) microemulsion synthesis and fiber formation


               
   3. Properties of alginate fibers with PCM                                                     
  • SEM morphology of calcium alginate fiber containing spherical PCM microemulsion size of around 3 μm, well dispersed in the emulsion, smooth surface of alginate fiber, figure.
  • Thermo-regulation alginate fiber had a comfortable phase change temperature of 25–35 ºC, and an acceptable phase change enthalpy of about 20 J/g.
B) Layering on fibers and flat product - finishing with binder and coagulant - method of impregnation and layering.

        Application of PCMs on fabric from Bamboo fibers 
1.  Preparation of PCMs:  
Modifiction of hydrofilicity of shell material with active monomer and 2-hydroxyethyl methacrylate (HEA) into methyl methacrylate (MMA) and butyl acrylate (BA) by copolymerization, see figure below. 


     
         2.) Application of PCMs
PCM microcapsules were applied onto bamboo fabric, by dipping into microcapsule emulsions (concentration of 25%) for 10 minutes and dried at a temperature of 80 ºC for 30 minutes. 

      
Morphology of phase change material microcapsule powders (spherical shape, size of 826 nm) (left); Untreated Bamboo fibers (middle); Treated Bamboo with PCM emulsions 

         3. Properties of Bamboo fabric by PCM
Bamboo is cellulosic fiber that has a large number of hydroxyl groups, as well as shell material of PCM capsules with HEA. The hydroxyl groups from both bamboo fiber and the shell material tended to form a hydrogen bond, which had very strong binding force.
Hydrophilicity of bamboo fabric after treatment was improved - contact angle is decreased from 69.3º to 32º, and thermal storage capacities was around 110 J/g.
Treated Bamboo fabric posses a better washing fastness, about 72% PCM microcapsules retained on fabric after 15 washing cycles. Owing to the better washing fastness, comfort of the resultant thermoregulating fabrics could be expected.

C) During the formation of foam and lamination - for the production of nonwovens; incorporation of microcapsules into the foam and incorporation into the textile structure by lamination or microcapsules into a thin polymer film and lamination on the inner part of the material.

   Lamination process of PCMs

For improving the thermo-physiological wearing comfort of protective garments, PCM are incorporated into a thin polymer film and applied to the inner side of the fabric system by lamination process. 

Beside chemical protective suits the PCM can also improve the thermo-physiological wearing comfort of other protective garments made of nonwovens such as surgical gowns, uniforms, or garments worn in clean rooms. Microcapsules would be mixed into a water-blown polyurethane foam mix and these foams are applied to a fabric in a lamination process, where the water is taken out of the system by drying process. The excellent honeycomb structure obtained during foam formation made considerable amount of still air trapping possibility, thus, leading to an increased passive insulation. 

   2. Different essential oils, vitamins, hydrating agents, antibiotics, disinfectants, dyes and pigments, etc. 

Microencapsulation using different ingredients as core material offers many opportunities to improve properties or provide entirely new functionalities of textile materials, see in table below.


                                                    Source: Coatings 2021, 11, 1371. https://doi.org/10.3390/coatings11111371

Effects of Microencapsulation in Functional Textiles

  • Permanent protection or separation of a core material for the life of the product. Such long-life microcapsules provide permanent functionalproperties of active thermal regulation or colour-changing textiles with electrochromic, photochromic and thermochromic materials (using PCMs).
  • Targeted release of the core - targeted release of active components from the core is envisaged, microcapsules with impermeable shells burst open by mechanical pressure, abrasion, melting or thermal decomposition. Until release, the active components in the microcapsule core remain separated from the reactive components (leuco dyes and colour developers), converted from a liquid to a solid state and protected against evaporation (essential oils) or protected against environmental influences and oxidation (essential oils, lipids and vitamins).
  • Long-lasting, gradual release by diffusion through the permeable microcapsule shell - principle is used in long-lasting perfumed textiles, in insect repellent fabrics and in sustained-release medical and cosmetic textiles.

Note: In the chapter 3. Functional properties read more about properties obtained with microcapsules. 

References:

Erkan, G., and Sariisik, M., Colourage, 51, 61 (2004)

Buschmann, H. J., and Schollmeyer, E., Mell. Textil., 84, 84 880 (2003)

Ahmed Hassan et al.: Sustainability 2016, 8, 1046; doi:10.3390/su8101046

Yan Wang et al.: Textile Research Journal, 2020, Vol. 90 (9–10) 1038–1044

Guoqing Zhang et al.: Textile Research Journal 2019, Vol. 89 (16) 3387–3393

Bojana Boh Podgornik et al.: Coatings 2021, 11, 1371. https://doi.org/10.3390/coatings11111371