Fabrication of surfaces with controlled wetting properties on porous substrates using non-fluorinated chemicals

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Tang, Zhenguan
Breedveld, Victor
Hess, Dennis W.
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Paper has traditionally been used for writing and printing, packaging and cleaning. Compared with products made with synthetic fibers, paper-based products offer many advantages such as low density, high flexibility, world-wide accessibility and good degradability. Due to these excellent properties, researchers are interested in further expanding the use of paper from its traditional roles to more advanced applications such as microfluidic devices and biological test strips. However, the intrinsic hydrophilicity and oleophilicity limit the potential applications of paper-based products. As a result, there is a continuous effort from both industry and academia to fabricate paper-based substrates with controlled wetting properties. However, although there is a plethora of existing methods to fabricate paper with a variety of modified wetting properties, most of these methods rely heavily on the use of fluorinated coating materials, which are often associated with significant environmental and health concerns. Therefore, investigation of methods to fabricate paper with controlled wetting properties using non-fluorinated chemicals is of significant interest. In this thesis, we utilize a variety of non-fluorinated coating materials to fabricate paper with controlled wetting properties including hydrophobic, amphiphobic and hydrophilic/oleophobic. To fabricate paper with amphiphobic properties, a non-fluorinated polymer, poly (benzoxazine), was used. Benzoxazine monomer was applied to paper surfaces by either a dip coating or drop casting method. The coated paper samples were subsequently cured at different conditions to initiate the polymerization process. The oleophobicity of PBZ coated paper samples is closely related to both the degree of polymerization of PBZ and the amount of PBZ on the paper. Paper samples with a loading of 10.6 mg/cm2 of PBZ display prolonged resistance against oils after curing at 180 oC for 12 hours. However, the disadvantages of PBZ as coating material are also obvious: high loading and prolonged curing time means that this procedure has limited economical feasibility. Treated paper also loses some flexibility due to the high-temperature-curing. To fabricate amphiphobic paper under milder conditions, a simple immersion coating method was developed to coat alkyltrichlorosilanes on paper surfaces under ambient conditions. Four different reagents were used in this study to investigate the effect of alkyl chain length on the wetting properties of alkyltrichlorosilane coated paper: methyltrichlorosilane (MTCS; -CH3), butyltrichlorosilane (BTCS; -C4H9), dodecyltrichlorosilane (DTCS; -C12H25) and octadecyltrichlorosilane (OTCS; C18H37). SEM analyses reveal that by systematically varying alkyl chain length, films with different surface morphologies could be generated on flat silicon wafer control samples and on cellulose-based paper samples. The variation in surface morphology leads to different wetting behavior, as determined by measuring static water and oil contact angles. Due to the nano- and micron- scale roughness on MTCS coated substrates, paper samples coated with MTCS display superhydrophobicity with a water contact angle of 152.2°, which is the highest water contact angle among these four alkyltrichlorosilanes. However, additional nano-scale roughness from an MTCS coating reduces the oil resistance of coated paper samples, while paper samples coated with long-chain alkyltrichlorosilanes have lower surface energy and lack nano-scale roughness. As a result, paper samples coated with OTCS display the highest resistance against oils (ethylene glycol contact angle, 125.5°; diiodomethane contact angle, 101.3°). Hydrophilic-oleophobic surfaces have attracted significant attention recently due to their potential use in technologies ranging from oil-water separation to self-cleaning surfaces. Studies with alkyltrichlorosilane coated paper demonstrate for the first time that a simple, solution-based method can be used to fabricate oleophobic paper with tunable hydrophilicity using a non-fluorinated material, methyltrimethoxysilane (MTMS). Wetting control is achieved by paper surface modification using a thin film of hydrolyzed MTMS. Hydrophilicity is tuned by adjusting the sonication time during MTMS hydrolysis. 29Si NMR and ATR-FTIR analyses reveal that the change in hydrophilicity results from varying the concentration of polar silanol groups in the MTMS solution and, ultimately, on the film surface. The modified paper displays wetting behavior ranging from superhydrophilic/oleophobic (immediate water absorption; motor oil contact angle, 64.2°±1.4°) to amphiphobic (water contact angle 85.2°±3.4°; motor oil contact angle 61.2°±2.5°) as a function of hydrolysis time. For all surface-modified samples, no absorption of motor oil is observed for several weeks, indicating stable oil resistance. Based upon results from SEM, optical profilometry, and air permeability, the intrinsic porosity of paper is also largely retained after coating. Application of the fundamental knowledge gained from studies of MTMS coated paper, allowed the fabrication of amphiphobic wood and hydrophilic/oleophobic stainless steel mesh surfaces using hydrolyzed MTMS. The amphiphobic wood has potential applications as a packaging material that can repel both aqueous and oily fluids, while the hydrophilic/oleophobic stainless steel mesh can be used to separate oil/water mixtures.
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