Issue 28/2012 - Research Progress on the Application of Shear Thickening Liquids in Protective Materials

In the mid-90s of the 20th century, under the guidance of Dr.
Eric Wetzel of the Army Research Laboratory (ARL) and Professor Norman Wagner of the Center for Synthetic Substances Research (UDTC) of Delaware State University, the United States successfully developed a new functional material using new nanotechnology - shear thickening liquid (STF).

It is a slightly viscous liquid in normal state, and when subjected to impact, the apparent viscosity will increase sharply, showing solid impact resistance, and when the impact force disappears, it quickly returns to the original flexible state
.

Liquid Armour, a soft protective composite material made of STF and high-performance fiber fabrics, is soft and comfortable, and can be used to make a full set of clothes with sleeves and trouser legs to protect any part of
the body.
STF makes fabrics resistant to bullets, bayonets and spikes without altering their weight, elasticity and comfort
.

First, the study of the protection mechanism of liquid protective composite materials

Relevant studies have shown that under the same areal density, compared with pure fiber fabrics, the bulletproof and anti-cone properties of liquid protective composites can be greatly improved, while the anti-knife performance is not greatly
improved 。 One theory that elastic resistance can be improved is that the application of STF strengthens the connection between filament-filament, bundle-bundle, and layer-layer in the fiber, thereby strengthening the overall response of the fiber to the impact force of the bullet, thereby improving the energy absorption rate; Another bulletproof mechanism is that the STF itself in the STF-composite material absorbs energy, and when the bullet directly hits the fabric, due to the shear action of the warhead or the mutual movement of the yarns in the fabric, the STF shears and thickens, thereby consuming energy
.

The improved cone resistance of STF-composite materials is likely to be due to the fact that after the fabric is impacted, under the action of shear force, the fabric can quickly collect the dispersed phase particles in the STF, fixing the fabric and reducing the ability to move between the fiber bundle and the filament, thereby preventing the cone tip from passing through
the fiber bundle or filament 。 In the anti-knife mechanism, the knife is mainly cutting effect on the fiber, and reducing the mobility between the fiber bundles or fiber filaments does not have a great impact on the anti-knife performance of the fiber, but from another point of view, the application of STF can actually improve the tightness between the fabric structures, so as to have a certain anti-puncture cutting ability
.

At present, there are not many research reports on the protection mechanism, and in order to design better liquid protective composite materials, further in-depth research
on the protection mechanism is needed.

Second, the application research of STF in protective materials

STF consists of
dispersed phase particles and dispersed media.
Among them, the dispersed phase particles can be naturally occurring minerals or chemically synthesized polymers, such as SiO2 and other oxides, polystyrene, calcium carbonate, polymethyl methacrylate (PMMA), etc.
, which are stably dispersed in the
medium solution by Brownian motion, charge action, absorption of surfactants, grafted polymers to form polymeric electrolytes, etc.
The dispersion medium may be water, organic matter (ethanol, vinyl ethanol or polyethylene glycol), salt solution (buffer or sodium chloride solution, etc.
), mineral oil, etc.
, or may be a complex ligand
of several miscible solvents added with oligomers and surfactants.

There are many factors affecting the performance of liquid protective materials, including fiber fabrics, STF dispersion systems, composite processes and composite structures, etc.
, and the main research is mainly the influence
of fiber fabrics and STF dispersion systems on material properties.

1.
Influence of fiber fabricAs one of
the important components of liquid protective composite materials, fiber fabrics have an important impact on
the overall performance of composite materials.
At present, the fiber fabrics compounded with STF mainly include Nylon, Kevlar and ultra-high molecular weight polyethylene (UHMWPE).

Egres Jr et al.
studied the puncture resistance of STF materials after compounding with Nylon and Kevlar, respectively, in which Nylon has three structures
: LD (low areal density high denier), MD (medium denier in areal density) and HD (high areal density low denier).
With the increase of fiber fabric denier, the anti-cone performance of STF-Nylon specimens is improved without much
impact on the anti-knife performance.
This is mainly due to the fact that as the denier value of the yarn increases, the thinner the yarn, the more yarn roots there are, and thus the more difficult it is to move between the fiber bundles; In addition, the finer the fabric yarn with high denier value, the lower the fabric areal density, so that the target material at the same areal density has more layers (such as HDNylon only 6 layers, while LDNylon target has 13 layers), and the increase in the number of layers increases the increase in the interlayer gap, so that the impact resistance of the target material is improved
.

In addition, it is found that STF-Nylon fabric has little dependence on yarn number and STF content during quasi-static puncture test, and the phenomenon of specimen damage after impact is not as obvious
as that of STF-Kevlar fabric.
This is because, compared to Kevlar, Nylon is easier to elongate and contract, so during the quasi-static puncture test, STF-Nylon fabrics are more likely to elongate rather than be punctured and cut due to the lower loading rate and
Nylon's large elongation.
And compared to STF-Kevlar fabrics, STF-Nylon fabrics are more likely to break the yarn than to be pulled out
because of their lower toughness.

2.
Influence of STF dispersion system In 2002, Dr.
Wagner first used SiO2/PEG200 STF dispersion system to treat Kevlar fiber fabric to prepare soft protective composites, studied the influence of STF volume and composite structure on material properties, and compared
the properties of
untreated pure Kevlar fabrics 。 The results show that with the increase of the STF mass fraction in the composite sample, the energy absorbed by the target material increases, in addition, when the 4-layer Kevlar fiber fabric is compounded with the same quality of STF, the protective performance is the best when the Kevlar fiber fabric is impregnated by STF.
If the same quality of dry SiO2 powder is added, its effect on the energy absorption of the target material is not as obvious as STF, and only the addition of PEG200 leads to a decrease in energy absorption, which indicates that it is STF that increases energy absorption, not SiO2 or PEG200
in STF.
At the same areal density, the energy absorption performance of the 4-layer STF-Kevlar composite is the same as that of the
14-layer pure Kevlar specimen.

Y.
S.
Lee et al.
studied the anti-elastic performance of Kevlar fabric impregnated by STF with reference to the American bulletproof standard N I Jstandard-0101.
04, in which the STF system was SiO2/PEG200, the particle size of SiO2 was 450nm, the volume fraction of SiO2 in the STF system was 57%, and the test bullet velocity was 244m/s
。 The results show that the energy absorption of STF-Kevlar composites is positively proportional to the volume of STF in the fabric.
Pure Kevlar fabrics of the same areal density and STF-Kevlar composites have the same elastic properties, but STF-Kevlar composite fabrics have fewer layers, so they are thicker and have better flexibility
.

Eric D.
Wetzell et al.
for the first time combined the experiment of STF rheological properties with the ballistic impact test of fiber fabrics to study the influence of different STF systems on the elastic performance of STF-Kevlar composites, the STF systems used are CaCO3/PEG200, SiO2/PEG200, of which CaCO3 is an elliptical particle (the aspect ratio is 2:1, 4:1, 7:1), and SiO2 is a spherical particle.
The results show that the elastic performance of STF-Kevlar composites is not as good as that of pure Kevlar fabrics when the particle volume fraction is low, and the elastic performance of the composites is better than that of pure Kevlar fabrics at higher volume fractions, but the particle parameters (such as length-diameter ratio) in the STF system have little
effect on the energy absorption of the composites.

Kalman et al.
studied the effect of monodisperse PMMA/PEGSTF system on the protective performance of fiber fabrics, and the results showed that regardless of whether the dispersed phase particles were PMMA or SiO2, the two composites had good anti-cone properties, and the anti-cone properties of the composite specimens did not change due to the change of dispersed phase particles.
Scanning electron microscopy observed the damage area of the composite material after impact, and found that PMMA particles had almost no destructive effect on the fabric, while SiO2 had a certain destructive effect on the fiber bundle.
Bulletproof tests show that the elastic resistance of composite specimens with dispersed phase particles PMMA is not as good as that of specimens with dispersed phase particles of SiO2
.
This difference may be due to the fact that hard SiO2 particles can invade the Kevlar fiber bundle to enhance the interaction between STF and fibers and fiber-to-fiber; Another reason may be that the elastic resistance of the material is related to the rheological properties of STF, and the thickening degree of PMMA/PEG system is not as good as that of SiO2/PEG system, and rheological tests show that there is a shear thinning region
in PMMA/PEG system at higher shear rates.

At present, there is little research on STF protective materials in China, and the research on this new liquid protective material is still in its infancy, and the research that has been carried out is not perfect
.

Third, the outlook

The application of STF in protective materials to make flexible protective materials can improve the shortcomings
of the current soft anti-stabbing clothing such as heavy weight, inflexibility, inconvenient walking, and inability to effectively protect some joint parts such as elbow joints, and sometimes even affect law enforcement personnel to perform official duties.
However, liquids with this shear thickening phenomenon usually adversely affect production due to irreversible flocculation, particle agglomeration, etc.
, such as obstructing pipelines and destroying production equipment
.
Therefore, it is necessary to study how to reduce the viscosity of STF to reduce negative effects on the one hand, and on the other hand, how to use this thickening performance of STF to turn harm into profit, so as to serve production, such as designing damping control equipment, researching the latest bulletproof materials or other protective equipment
.