Characterization of hydrogel antifouling materials

Marine biofouling refers to the harmful attachment and growth of marine organisms on underwater facilities

Hydrogel is a three-dimensional network material formed by physical or chemical cross-linking of hydrophilic macromolecules and water.

1 Characterization of general properties of hydrogel antifouling materials

1.

The swelling property of the hydrogel is manifested as the increase of the volume of the hydrogel after absorbing the solvent

In the formula: md—the mass of the sample when it is dry; ms—the mass of the sample when it is immersed in the solvent and the mass reaches equilibrium (it is necessary to absorb the moisture on the surface of the sample with filter paper and weigh it)

The method refers to GB/T16886.

1.

The low elastic modulus of the hydrogel antifouling material makes its surface continuously deformed under the impact of water flow and presents an unstable state, which has an adverse effect on the attachment of fouling organisms and is beneficial to improve the antifouling of hydrogel.

The principle of measuring Young's modulus by the tensile method is to fix one end of the sample with a tensile machine, and hang the other end vertically and apply a certain tensile force, and calculate the Young's modulus of the material by measuring the relationship between stress and strain

1.

The polymer surface with microphase-separated structure has been proved to have excellent anticoagulant properties, and the surface microphase-separated morphology of sharkskin also makes sharkskin have considerable antifouling ability

Recently, Hu et al.

Figure 1 Fluorescence imaging of microphase-separated hydrogels

The microphase separation structure on the hydrogel surface can not only reduce the adhesion of fouling organisms by reducing the focal point of fouling organisms, but also increase the contact angle of the hydrogel surface, which is more conducive to preventing fouling organisms
.
This has certain commonalities with the antifouling mechanism of low surface energy antifouling coatings
.
The surface energy and contact angle of the hydrogel antifouling material can be measured by a contact angle meter
.
However, the performance research of hydrogel antifouling materials is generally carried out in seawater system, and the inorganic salt ions on the surface of hydrogel antifouling materials balanced in seawater will affect the measurement of the contact angle of the materials
.
Therefore, when investigating the surface energy of the hydrogel antifouling material, it needs to be soaked in deionized water
.
After measuring the contact angle of the hydrogel antifouling material, the surface energy of the material can be calculated using the Owens two-liquid method, as shown in formula (2)
.

In the formula: γL—liquid surface tension; θ—contact angle; γdS—dispersive force component of solid surface energy; γdL—dispersive force component of liquid surface tension; γpS—polar force component of solid surface energy; γpL—liquid surface tension the polar force component
.

2 Characterization of application properties of hydrogel antifouling materials

2.
1 Adhesion performance on the substrate surface

The hydration layer on the surface of the hydrogel material makes it difficult to adhere to the surface of the substrate, which affects the practical application
.
Therefore, the improvement of the adhesion performance of hydrogel antifouling materials on substrates is a current research hotspot
.
The adhesion ability of the coating on the substrate is expressed by the adhesion, which is often tested by a universal tensile machine
.
Lu et al.
used the adhesion strength (the measured maximum load divided by the adhesion area) to characterize the adhesion performance of the hydrogel when investigating the high adhesion performance hydrogel containing catechol structure
.
Liu et al.
used the peel-adhesion strength test to characterize the hydrogel adhesion properties, and used a texture analyzer to perform a 90° peel test on the hydrogels adhered to various solid surfaces
.
Both of the above two methods put forward requirements on the strength of the hydrogel material itself, that is, when the adhesion force of the hydrogel on the substrate is less than the strength that the material can withstand, the pull-off method can be used
.
However, for the hydrogel used to prevent marine biofouling, in order to ensure the low elastic modulus of the material (low elastic modulus helps to increase the antifouling ability of the hydrogel), the tensile force that it can withstand is generally small, so The tensile method is not suitable for determining the adhesion properties of hydrogel antifouling materials
.
For the same reason, the paint film adhesion test standard GB/T5210-2006 "Paint and varnish pull-off method adhesion test" is not suitable for measuring the adhesion of hydrogel antifouling materials.
Determination
.

The adhesion of the coating can also be evaluated by the circle method and the method of scratching the surface of the coating with a hundred grid knife
.
Both methods are used to evaluate the peeling condition of the coating surface.
The measurement method is simple, intuitive and effective, and is widely used to evaluate the adhesion of dry coatings
.
However, compared with ordinary coatings, hydrogel coatings are wet and have certain elasticity and viscosity
.
Therefore, the adhesion ability of the hydrogel coating on the surface of the substrate is not suitable to be evaluated by the above two methods
.

It can be seen that the traditional coating adhesion test methods cannot effectively, simply and intuitively evaluate the adhesion performance of the hydrogel antifouling material on the substrate
.
According to the characteristics of the hydrogel antifouling material and the actual application environment, a combination of static immersion and dynamic paddling can be used to evaluate the adhesion ability
.
The method is mainly divided into two steps: first, the hydrogel antifouling coating samples are preliminarily screened in seawater by the "static immersion" method, and the hydrogel coating samples with better adhesion are selected for further testing; then , through the "dynamic paddling" to further investigate the samples that were initially screened
.
In the "dynamic paddling" stage, the "rating" is made by comprehensively considering the peeling time and peeling area of ​​the hydrogel coating
.
Among them, the static immersion refers to GB/T10834-2008 "Determination of Salt Water Resistance of Marine Paints in Salt Water and Hot Salt Water Immersion Method", using ABS board as the base material, the prepared hydrogel coating sample is immersed in seawater, and the sample is immersed in seawater for 7 days.
The hydrogel coating that peeled off was rated as level 1, and the hydrogel coating that did not peel off after 7 days was rated as level 2; dynamic paddling was based on GB/T7789-2007 "Dynamic Test Method for Antifouling Performance of Ship Antifouling Paints".
The coating sample is fixed on the sample frame of the dynamic test device, simulates the seawater scouring environment at the specified speed, and observes the surface shedding of the hydrogel antifouling coating after a certain period of time, and integrates the two factors of time and shedding area.
Adhesion was rated
.
By combining static immersion and dynamic paddling, the adhesion ability of hydrogel antifouling materials on substrates and in the actual marine environment can be evaluated very effectively, intuitively and simply
.

2.
2 Polishing performance

The antifouling function of hydrogel antifouling materials can be used, in addition to the use of the hydrated layer of the material itself and the unstable surface caused by low elastic modulus, the hydrogel material can also be slowly hydrolyzed and polished to simulate the mucus secreted by organisms.
Further increase the instability of the hydrogel surface to enhance the antifouling performance of the hydrogel antifouling material
.
The self-polishing performance of antifouling coatings is generally expressed by the abrasion rate.
During the test, the sample is installed on a rotating mold submerged in seawater, and the abrasion test is performed at a certain speed to test the change of coating thickness per unit time
.
For the hydrogel antifouling material, its unique swelling characteristics make the thickness change of the coating not obvious before and after accelerated abrasion.
Better reflect the polishing performance of hydrogel antifouling materials
.
The polishing performance of the hydrogel can be expressed by the polishing rate (D), as shown in Equation (3)
.

In the formula: m0—the quality of the empty sample plate; m1—the quality of the sample plate before soaking; mt—the quality of the sample plate after soaking
.

The polishing rate test method generally adopts the drum method, and the specific methods, conditions and paddling facilities refer to GB/T31411-2015 "Determination of Abrasion Rate of Ship Antifouling Paint"
.

3 Characterization of antifouling properties of hydrogel antifouling materials

3.
1 Anti-protein adsorption performance

Biofouling on the surface of marine facilities usually undergoes multiple steps such as the formation of organic molecular layers, the formation of biofilms, the attachment of algal spores and protozoa, and the attachment of large fouling organisms, as shown in Figure 2
.
Among them, the adsorption of organic substances in seawater, such as proteins, polysaccharides, nucleic acids, etc.
, on the surface of objects is a key step in the occurrence of fouling
.
One of the mechanisms by which the hydrogel exerts its antifouling effect is to use the hydration layer on the surface to inhibit the adsorption of organic molecules, thereby inhibiting the further attachment of subsequent fouling organisms
.
Therefore, the investigation of the anti-protein adsorption ability of the hydrogel surface is an important part of the evaluation of the antifouling performance of the material, and it is also an important aspect that is different from the investigation of the antifouling ability of traditional antifouling materials
.

Figure 2 The formation process of biofouling

The adsorption of proteins on the material surface can be characterized by quartz microcrystalline balance (QCM) and surface plasmon resonance (SPR)
.
QCM characterizes the change of the sensor surface quality by detecting the change of the vibration frequency of the quartz oscillator.
It is a very sensitive detector, which can dynamically detect the quality of protein adsorption, the thickness of the adsorption layer, the change of viscoelasticity and the adsorption of protein in real time.
conformational changes,
etc.
SPR is a bio-optical detection device designed using the principle of plasmon resonance.
It can be used to detect the interaction between various biomolecules, including the adsorption of proteins on the surface.
It has the advantages of no fluorescent labeling, real-time detection, and high sensitivity
.
If there is no need to pay attention to the adsorption process of proteins on the surface of hydrogel materials, Coomassie brilliant blue spectrophotometry (Bradford method) can also be used to determine the amount of protein adsorbed on the contaminated surface
.
Coomassie brilliant blue is red in the free state and has a maximum light absorption at 488nm; when it binds to a protein, it turns cyan and has a maximum light absorption at 595nm
.
The light absorption value is proportional to the protein content, so it can be used for quantitative determination of protein
.
The method was established by Bradford in 1976.
It is simple, quick to operate, sensitive to response, and can measure microgram-level protein content (the mass concentration range of protein is 0~1000μg/mL, and the minimum can be measured 2.
5μg/mL).
It is a commonly used trace Methods for the rapid determination of proteins
.
The core of the research on the anti-protein adhesion ability of hydrogel antifouling materials is the amount of protein adsorbed on its surface per unit time, and the adsorption process of protein is not concerned
.
Based on the consideration of efficiency and demand, Coomassie brilliant blue method is a suitable method for evaluating the anti-protein adhesion ability of hydrogel antifouling materials
.

The commonly used proteins are immunoglobulin G (IgG) and bovine serum albumin (BSA) in the investigation of the adsorption of proteins on the surface of the material.
The response of IgG to Coomassie brilliant blue reagent is twice that of BSA
.
Therefore, IgG is more suitable for the experimental study of hydrogel anti-protein adsorption
.
There are some substances that interfere with the determination of this method, such as detergents, TritonX-100, sodium dodecyl sulfate (SDS), and a strong alkaline environment
.
If the sample contains corresponding interfering substances, it is necessary to eliminate the interfering substances before testing
.

3.
2 Algae inhibition performance

Microalgae are an important component of the microbial mucosa formed during the development of marine biofouling, so it is necessary to investigate the algae-inhibiting ability of hydrogel antifouling materials
.
Because there is no corresponding algae inhibition test standard for water-insoluble chemicals, Liu Hong et al.
immersed the coating samples in a liquid culture containing diatoms for a certain period of time, and then completely brushed off the algae attached to the coating surface, and then measured the washing.
The algal biomass in the liquid was used to evaluate the antifouling performance of the coating
.
The test cycle and equipment can refer to GB/T21805-2008 "Chemical Algae Growth Inhibition Test".
The algal biomass can be measured by cell count as an alternative parameter, measured by microscope, hemocytometer and other instruments, or by spectrophotometer, Fluorometers, etc.
to measure other alternative parameters
.
By comparing the absorbance method, the fluorescence method and the microscope counting method, Zhang Nahui et al found that the absorbance method has the problem that the detection limit is not low enough, and the debris produced by the dead algal cells will interfere with the absorbance and cause measurement errors
.
The fluorescence counting method has a lower detection limit and the detection results are more correlated with the microscopic counting results.
Therefore, based on the chlorophyll fluorescence parameter, Chen et al.
determined the algal biomass on the coating surface through the linear relationship between the fluorescence value and the algae concentration, and evaluated the water The algae inhibition performance of the gel antifouling coating has better applicability and accuracy
.

3.
3 Real sea hanging board test

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4

Compared with traditional antifouling materials, hydrogel antifouling materials have special antifouling mechanism and physical and chemical properties, and generally have poor mechanical properties and adhesion to substrates, dehydration and brittleness, long-term antifouling and poor adhesion.
Problems such as lack of broad spectrum
.
This makes the properties of such materials to be investigated and concerned are different from those of traditional antifouling coatings
.
The swelling property of the hydrogel can be characterized by the weighing method; the elastic modulus can be characterized by the method of measuring the Young's modulus of the hydrogel antifouling coating by the indentation method, which is relatively easy to operate and has high accuracy; low surface area The hydrogel antifouling material can be characterized by measuring the contact angle and then using the Owens two-liquid method.
The special microphase separation structure can be characterized by laser confocal microscopy, fluorescence microscopy, environmental scanning electron microscopy, and freeze-drying.
The samples were prepared by SEM and AFM, and the surface morphology was observed by SEM and AFM; since the hydrogel antifouling coating is not suitable for the adhesion performance test method of the traditional antifouling coating, the static immersion combined with the dynamic paddling test can be used.
The adhesion performance of the coating was rated by the peeling condition of the surface of the coating; the polishing performance was measured by the weighing method; in terms of antifouling performance, the Coomassie brilliant blue method could be used to characterize the anti-protein adhesion performance of the hydrogel antifouling coating.
The number of microalgae attached to the surface of the coating is used to characterize the algae-inhibiting performance of the hydrogel antifouling coating.
In the actual sea hanging plate test, the hydrogel antifouling material is the same as the traditional antifouling material, and can be characterized with reference to national standards
.

Source of this article: 2021 "Coatings Industry" Issue 9

Authors: Jia Ning, Dai Shuaishuai, He Guangling, Li Changcheng, Zhang Xia, Dong Lei, Yu Liangmin