The surface modification of ballistic textiles using plasma-assisted chemical vapor deposition (PACVD)

Abstract

This paper describes studies on the surface modification of so-called ballistic materials (materials commonly used to protect the human body against firearms, i.e. fragments or bullets). Two materials, an ultra-high molecular weight polyethylene (UHMWPE) composite and aramid fabric, were investigated. The surfaces of these fibrous materials were modified using plasma-assisted chemical vapor deposition (PACVD) to examine the effects of the modification on the material properties, which are important for designing ballistic protections. Accordingly, both the mechanical strength and water resistance of the modified materials were tested. The results clearly show the impact of the modification on both parameters.

Keywords

surface modification material strength material properties PACVD chemical modification composites

Over the last 20 years, most of the literature reports on the modification of ballistic material surfaces by plasma-assisted chemical vapor deposition (PACVD) have focused on the modification of basic raw materials—fibers, yarns or purified fabrics.^1–6,8 Experiments on these materials focused on improving the adhesion (e.g. of ultra-high molecular weight polyethylene (UHMWPE) to epoxy resin or low-molecular weight polyethylene as a ballistic material matrix).^12,13 Improved adhesion allows the modified fibers and matrix to form a more stable connection.

Low-temperature plasma techniques are superior to the currently used finishing techniques because they do not require large quantities of water or toxic processing aids and are therefore more environmentally friendly. Additionally, the plasma modification process is significantly more efficient economically and technologically than standard dyeing and finishing processes. In a relatively short time, the surface properties of the material can be significantly altered in a controlled manner to improve the wetting, adhesion, oleophobic resistance or superhydrophobicity, changing the physical characteristics, or to sanitize and disinfect the large surfaces of the materials.⁹

Moreover, the modification of aramid fibers with NH₃ plasma and O₂ plasma was studied. The application of these two gases improved aramid fiber adhesion to epoxy resin without significantly affecting the mechanical strength (approx. 10%).⁷ Additionally, the effects of modifying purified aramid fabrics with plasma on pre-impregnates constructed from the modified fibers and phenol resin were investigated.¹⁰ The structural changes in the aramid fibers due to the plasma modification were investigated with XPS and FT-IR, and their morphology was examined with SEM. During the fiber surface modification, oxygen and hydrogen functional groups were created. As a result, the adhesion of the p-aramid fabric to the phenol resin improved. Additionally, resistance to abrasion and delamination of the terminal pre-impregnates (prepregs) was observed.

The use of argon in the plasma modification of UHMWPE resulted in changes in its wettability and resistance to mechanical damage at the surface. The results show that the application of this gas improved properties that are fundamental to the production of ballistic components.¹¹

The inclusion of wool in ballistic materials can significantly improve the tear strength of pure synthetic ballistic fabrics.¹³

Because the fibers are fundamental elements of the final fabric structure, their surface modification resulted in abrasion during the waving process and thus led to some problems during textile fabrication. The same phenomenon was observed during the preparation of an UHMWPE fiber composite when the fibers were incorporated into the polymer matrix (polyisoprene binder and polyethylene film).^12,14

Previous research focused only on the surface modification of single fibers to improve the affinity of the high-strength fibers to resin for use in the fabrication of prepregs. The structure of fibrous materials, especially fabrics or fibrous composites, is significantly more complex than that of others (i.e. films, foils). Thus, the functionalization processes for fibrous materials are more complicated, especially in terms of their consistency and durability.

The aim of this study was to investigate the effects of low-temperature plasma treatment in the presence of two low molecular mass organic derivatives containing fluorine or silicon on the most critical properties of two types of ballistic raw materials, a p-aramid woven fabric and UHMWPE fiber composite, for future applications.

Materials and methods

This work investigated the influence of the surface modification of two different materials by PACVD on their properties (especially those critical for ballistic applications). Specifically, the materials studied were Kevlar® (p-aramid woven fabric; SAATI/Spain; surface density acc. the PN-ISO 3801:1993 Standard—212.0 ± 2.0 g/m²; thickness acc. the PN-EN ISO 5084:1999 Standard—0.20 ± 0.02 mm) and the Dyneema® SB51 composite (ultra-high molecular weight polyethylene; DSM/The Netherlands; surface density acc. the PN-EN ISO 2286-2:1999 Standard—251.0 ± 2.0 g/m²; thickness acc. the PN-EN ISO 2286-3:2000 Standard—0.19 ± 0.02 mm). The soft ballistic Dyneema® composite consists of a low molecular weight polyethylene (LMWPE) and polyisoprene binder matrix that contains UHMWPE fibers.^12,13 The soft ballistic Dyneema® composite is composed of approx. 70 wt% UHMWPE fibers and 30 wt% LMWPE and binder.¹²

Both materials are usually used to design soft ballistic inserts for bullet- and fragment-proof vests.

The surfaces of these materials were modified using a low-temperature, radio-frequency (13.56 MHz) plasma generator as depicted in Figure 1.

Figure 1.

PACVD system.

The PACVD system (CD 400 PLC R/R model; Europlasma, Belgium) consisted of two parallel rectangular aluminum electrodes. The right one was connected to an alternative voltage (radio frequency—13.56 MHz), whereas the left one was grounded. The surface to be modified was placed between the electrodes, facing the right one. During the process, the material to be deposited was injected (in the gas phase) into the chamber simultaneously with the gas used for the plasma initiation.

The surface modification of both Kevlar® and Dyneema® was performed in five stages: (a) proper evacuation of the chamber, (b) surface cleaning, (c) surface activation, (d) deposition of the material on the surface,(e) conditioning of the deposited layer. The appropriate conditions (gas type, gas stream, stage duration, deposited material, chamber pressure, electric power required for the initiation and continuation of the plasma treatment, etc.) must be determined for each stage.

To modify both Kevlar® and Dyneema®, two organic compounds, namely hexamethyldisiloxane (HMDSO) (C₆H₁₈OSi₂; molecular mass: 162.38 g/mol; Sigma Aldrich) and tetradecafluorohexane (TDFH) (C₆F₁₄; molecular mass: 338.04 g/mol; Tokyo Chemical Industry), were used.

After the surface modification of both raw materials, their properties were measured and compared to those of the unmodified materials.

Two gases, argon and air, were used to modify the Kevlar® woven fabric surface. The process stages in which they were used are given in Table 1. For the HMDSO deposition, the compound was injected into the chamber simultaneously with argon.

Table 1.

Stages of the surface modification of the Kevlar® woven fabric and Dyneema® SB51

Deposited material	HMDSO	TDFH
Surface cleaning	Ar	Ar
Surface activation	Air	Air
Deposition	Ar + C₆H₁₈OSi₂	C₆F₁₄

Two gases, argon and air, were used to modify the Dyneema® SB51 surface. The process stages in which they were used are given in Table 1.

The mechanical resistances of the unmodified and modified surfaces were tested according to the following standards:

For the Kevlar® woven fabric: (a) tear resistance acc. the PN-EN ISO 13937-2:2002 Standard; (b) tensile strength and elongation at the maximum force acc. the PN-EN ISO 13934-1:2002 Standard; (c) bursting strength acc. the PN-EN 863:1999 Standard.

For the Dyneema® SB51 composite: (a) tear resistance acc. the PN-EN ISO 4674-1:2005 Standard; (b) tensile strength and elongation at the maximum force acc. the PN-EN ISO 1421:2001 Standard; (c) bursting strength acc. the PN-EN 863:1999 Standard.

The testing methods for the fabrics (Kevlar®) and fibrous composite materials (Dyneema®) were selected based on the material specifications in the standards for a range of applications and our experience in testing of a wide range of ballistic materials. Moreover, the testing methods selected for the Dyneema® composite allowed the real mechanical behavior of the material to be determined.

The first two tests were performed in the two perpendicular directions, namely along the warp and weft of the fabric.

Additionally, the water resistance of the materials was examined because it indirectly affects the long-term usability of ballistic products. Two parameters were evaluated: (a) the resistance to surface wetting (spray test) acc. the PN-EN 24920:1997 Standard and (b) the water repellency of the fabrics during the Bundesmann rain-shower test acc. the PN-EN 29865:1997 Standard. Moreover, the resistance of Dyneema® SB51 to water penetration was tested acc. the PN-EN 20811:1997 Standard.

Results and discussion

Modification of the Kevlar® woven fabric

Tear resistance

Both HMDSO and TDFH deposition clearly result in a reduction in the tear resistance of the modified materials relative to that of the unmodified material. The differences in measured parameter are more significant when the fabrics are tested along the weft and when TDFH is deposited (approx. 60%). However, the surface modification with HMDSO also results in a similar decrease in the measured properties by approx.53% (Figure 2).

Figure 2.

Tear resistance of the unmodified Kevlar® woven fabric and Kevlar® modified with HMDSO or TDFH.

Moreover, observations of the tearing after the test show that the detected effect is the result of looser yarns instead of torn ones. The macroscopic imaging of the tearing area confirms the absence of torn yarns (Figure 3). The increased looseness of the woven structure is a direct result of the plasma modification process.

Figure 3.

Kevlar® woven fabric modified with HMDSO by low-temperature plasma after the tear resistance test.

The loosening of the p-aramid woven fabric structure after the PACVD modification might affect the ballistic behavior of the designed soft inserts, and the effects of the surface modification on their behavior should be investigated in the next stage of the study.

Tensile strength and elongation at the maximum force

Both HMDSO and TDFH deposition lead to a significant increase in the tensile strength of the modified materials relative to that of the unmodified material (Figure 4). Similar to the tear resistance results, the differences are more significant when the tests are performed along the weft of the fabric. In contrast to the tear resistance results, the tensile strength increases when TDFH is deposited. Specifically, the low-temperature plasma treatment with TDFH leads to an increase of approx. 40% in the tensile strength, while the modification with HMDSO results in an increase of approx. 26%. The elongation results depend on both the stretching force direction and deposited compound. When the force is applied along the warp of the fabric, the effects of both the HMDSO and TDFH modifications are the same, i.e. the elongation parameter increases slightly (by approx. 4%) (Figure 5).

Figure 4.

Tensile strength of the unmodified Kevlar® woven fabric and Kevlar® modified with HMDSO or TDFH.

Figure 5.

Elongation at the maximum force of the unmodified Kevlar® woven fabric and Kevlar® modified with HMDSO and TDFH.

When the stretching force acts along the weft of the fabric, the modification with TDFH leads to a slight increase in the elongation (by approx. 2%), while the elongation decreases by approx. 6% when the surface is modified with HMDSO.

Bursting strength

The bursting strength results clearly show that one of the deposited compounds strengthens the fabric whereas the other weakens it. As shown in Figure 6, the HMDSO deposition causes a significant increase in the bursting strength (by approx. 50%), whereas the application of TDFH causes a decrease of approx. 20% in the bursting strength. The changes in the bursting strength are significant relative to the bursting strength of the initial raw material, indicating that the relatively short low-temperature plasma treatment significantly alters the mechanical properties of the p-aramid woven fabric.

Figure 6.

Bursting strength of the unmodified Kevlar® woven fabric and Kevlar® modified with HMDSO and TDFH.

Water repellence

The wetting resistance of the fabric was studied by measuring the average water permeability and wettability. The results show that both TDFH and HMDSO deposition have little effect on its wettability as shown in Figure 7.

Figure 7.

Water absorption of the unmodified Kevlar® woven fabric and Kevlar® modified with HMDSO and TDFH.

On the other hand, the low-temperature plasma treatment in the presence of HMDSO or TDFH has a positive effect on the average water absorption, i.e. both compounds significantly reduce the water absorption. The Kevlar® woven fabrics modified with TDFH and HMDSO showed significant decrease of the water throughput as compared to the unmodified fabric (Figure 8). This observation suggests that if the modified materials are used in the final ballistic panels, the resistance of the fabric to water penetration during standard use will be enhanced.

Figure 8.

Average water throughput of the unmodified Kevlar® woven fabric and Kevlar® modified with HMDSO and TDFH.

Resistance to surface wetting

As shown in Table 2, the low-temperature plasma treatment with TDFH or HMDSO increases the surface wetting resistance.

Table 2.

Surface wetting resistance of the unmodified Kevlar® woven fabric and Kevlar® modified with HMDSO and TDFH

Initial materials	Low-temperature plasma treatment in the presence of:
Initial materials	HMDSO	TDFH
1; 1; 1	3; 2; 2	4; 4; 3

1—the lowest resistance to wettability; 5—the highest resistance to wettability.

The low-temperature plasma treatment with TDFH results in a significant reduction in the wettability of the modified surface of the p-aramid woven fabric. The increases in the tested parameters are lower when the woven fabric is modified with HMDSO. The low-temperature plasma treatment also reduces the water throughput of the materials considerably.

Modification of the Dyneema® composite

Tear resistance

The initial sample and composites after modification via the low-temperature plasma treatment exhibit no tearing at the test conditions (acc. PN-EN ISO 4674-1:2005 Standard) as shown in Figure 9.

Figure 9.

Dyneema® composite modified with TDFH by low-temperature plasma after the tear resistance test.

Due to the complex, compacted structure of the Dyneema® composite, no structure loosening is observed, unlike the results for both the unmodified and modified Kevlar® fabrics. The low-temperature plasma treatment does not lead to changes in the tear resistance of the UHMWPE composite, which is one of the main parameters affecting its ballistic performance.

Tensile strength and elongation at the maximum force

The deposition of TDFH increases the tensile strength of the modified material relative to that of the unmodified material.

However, the modification with HMDSO leads to a reduction in the tensile strength. This phenomenon occurs along both the longitudinal and vertical directions, but the impact of the surface modification is most clearly observed along the longitudinal direction, where the tensile strength changes by approx. 8% (TDFH) and 22% (HMDSO) (Figure 10).

Figure 10.

Tensile strength of the unmodified Dyneema® SB51 composite and Dyneema® SB51 composites modified with HMDSO and TDFH.

The elongation at the maximum force depends on both the stretching force direction and type of deposited compound. The deposition of TDFH reduces the elongation in both the longitudinal and vertical directions. However, the decrease in the elongation is most clearly observed in the vertical direction (by approx. 17%).

On the other hand, the modification with HMDSO results in a significant increase in the elongation along the longitudinal direction (by approx. 17%), while no change in the elongation is observed when the measurement is performed in the vertical direction (Figure 11).

Figure 11.

Elongation at the maximum load of the unmodified Dyneema® SB51 composite and Dyneema® SB51 composites modified with HMDSO and TDFH.

Bursting strength

The bursting strength results clearly show that the modification with both deposited compounds leads to a slight weakening of the composite. As shown in Figure 12, the HMDSO deposition results in a slight reduction in the bursting strength (average by approx. 15%). However, due to statistical deviations, the bursting strengths of the unmodified composite and composite modified with HMDSO are comparable. The modification with TDFH results in amore significant decrease in the studied parameter (by approx. 66%).

Figure 12.

Bursting strength of the unmodified Dyneema® SB51 composite and Dyneema® SB51 composites modified with HMDSO and TDFH.

Water repellence

The water resistance results show that both TDFH and HMDSO deposition (especially TDFH) affected slightly on the water repellence of the composite (Figure 13). The increase in the average water absorption value was increase for TDFH modified materials by approx. 40%, whereas the modification using HMDSO yielded in increase by approx. 20% as compared with the raw materials. Above phenomenon can be explained by the increase in the surface enhance of the modified materials influencing slight increase in the water particles capturing onto the composite surface.

Figure 13.

Water absorption of the unmodified Dyneema® SB51 composite and Dyneema® SB51 composites modified with HMDSO and TDFH.

Moreover, both modifications impact the resistance of the tested materials to water penetration. The TDFH deposition leads to an increase of approx. 25% in the tested parameter, while the modification with HMDSO results in an increase of approx. 39% (Figure 14).

Figure 14.

Resistance to water penetration of the unmodified Dyneema® SB51 composite and Dyneema® SB51 composites modified with HMDSO and TDFH.

Resistance to surface wetting

The surface modification with TDFH or HMDSO during the low-temperature plasma treatment results in an increase in the surface wetting resistance of Dyneema® SB51 in both cases (Table 3).

Table 3.

Surface wetting resistance of the unmodified Dyneema® SB51 composite and composites modified with HMDSO and TDFH

Initial materials	Low-temperature plasma treatment in the presence of:
Initial materials	HMDSO	TDFH
3; 3; 3	4; 4; 4	4; 4; 4

1—the lowest resistance to wettability; 5—the highest resistance to wettability.

The observed changes in the surface wettability relative to that of the unmodified Dyneema® SB51 are smaller than those observed for the modified p-aramid woven fabrics. Moreover, the volume of water absorbed by the modified UHMWPE composites is similar to that absorbed by the TDFH-modified p-aramid woven fabric and lower as compared with HMDSO modified p-aramid woven fabric (Table 2).

Conclusions

The effects of the PACVD modification of ballistic raw materials on their mechanical strength and water resistance were investigated. The results show that the appropriate choice of the deposited compound and selection of the process conditions can significantly change the surface properties of the initial (unmodified) material. A deposited thin layer of an organic compound increases the minimal stretching force required to mechanically damage the material. This layer can also transform the percolating textile into a nearly waterproof one. However, some of the mechanical strength parameters tested are not sensitive to the surface modification or are negatively impacted by it.

Until now, research has focused only on the modification of simple systems—single aramid fibers or UHMWPE fibers.^1–8,10 This work assumes that the fibers undergo a simple modification (as a preliminary attempt to assess the impact of modifying complex systems, namely UHMWPE fibers in a polymer matrix (Dyneema®) and raw aramid fabrics that were not initially purified). During the PACVD process, the initial purification stage used to prepare the surface significantly influences the main polymer layer deposition process. Including the purification process in the modification procedure shortens the overall process and makes it economically feasible.

The low-temperature plasma treatment leads to either a slight decrease or no change in the mechanical strength of the ballistic materials. UHMPWE composite materials (i.e. Dyneema® or Spectra®) are relatively sensitive to high temperatures, especially those above their melting temperatures. The tensile strengths of Kevlar fabrics modified with fluorine- or silicon-containing organic compounds are greater than that of the unmodified Kevlar. In contrast, the tensile strengths and bursting strengths of the modified Dyneema® composites, especially the composite modified with the silicon-containing compound, are smaller than those of the unmodified composite. These results emphasize the greater sensitivity of the UHMWPE matrix to the modification process conditions. The effect of the decrease in the mechanical strength of the modified UHMWPE composite on the ballistic performance will be studied to determine the correlation between the mechanical and ballistic behaviors of the modified materials.

The results indicate that the use of PACVD to deposit a polymer layer on the fiber (Kevlar®) and UHMWPE composite (Dyneema®) surfaces might significantly improve the important material properties:

Water resistance of the p-aramid fibers is improved (degradation from water exposure is a main factor leading to a loss in the fiber strength, which eventually weakens the ballistic behavior). Consequently, the ballistic protections can be used for a longer period of time without a significant loss of product performance and safety.

The surface properties of both studied ballistic materials can be changed in a controlled way, which is helpful for, e.g., the fabrication of a hybrid composite consisting of more than one type of ballistic or non-ballistic material. The combination of materials with different performances might significantly improve the main functionality of the fabricated composites and provide them with new functionalities.

Moreover, the outcomes of this work show that PACVD is promising for ergonomics studies and the transformation of textiles such as Kevlar® and Dyneema® into materials used for protection against firearms.

The results presented here are initial tests, and terminal ballistic tests will be performed in the near future. In the next stage of this research, the effects of the PACVD modification on the ballistic behavior (resistance to fragments and bullets acc. the PN-V-87000 Standard for a soft ballistic system optimized for the K2 bullet-proof class and O2 fragment-proof class) and stability of the modification effects during accelerated aging studies (aging factors: temperature, humidity, UV light) will be studied. Moreover, the PACVD modification of the ballistic raw materials with HDMSO and TDFH will be described using ATR-FTIR spectroscopy, DSC and SEM-EDS. The topography of the modified fibrous materials will also be studied using SEM microscopy.

References

Breznick

Banhaji

Guttmann

. Surface Treatment Technique for Aramid Fibers. Polym Commun 1987; 28: 55–55.

Andrepoulos

. A New Coupling Agent for Aramid Fibers. J Appl Polym Sci 1989; 38: 1053–1053.

Holmes S, Schwartz P. Amination of Ultra-High Strength Polyethylene using Ammonia Plasma. Compos Sci Technol 1990; 38.

Tissington

Pollard

Ward

. A Study of the Effects of Oxygen Plasma Treatment on the Adhesion Behaviour of Polyethylene Fibers. Compos Sci Technol 1992; 44: 185–185.

Gao

Zeng

. Surface Modification of Ultra-high Molecular Weight Polyethylene Fiber; Part I. Improve Surface Adhesion. J Appl Polym Sci 1993; 47: 2093–2093.

Tarantili

Andreopoulos

. Mechanical Properties of Epoxies Reinforced with Chloride Treated Aramid Fibres. J Appl Polym Sci 1997; 65: 267–267.

Brown

Mathys

. Plasma Surface Modification of Advanced Organic Fibres: Part 5. Effects on the Mechanical Properties of Aramid/Phenolic Composites. J Mater Sci 1997; 32: 2599–2599.

Nakajima

Koh

Singh

. Electrochemical Behavior of Surface-fluorinated Graphite. Electrochim Acta 1999; 44: 2879–2888.

Morent

De Geyter

Verschuren

. Non-thermal Plasma Treatment of Textiles. Surface & Coatings Technology 2008; 202: 3427–3449.

10.

Fang Guo Zhao-Zhu Zhang Wei-Min Liua . Effect of Plasma Treatment of Kevlar Fabric on the Tribological Behavior of Kevlar/Fabric Phenolic Composites. Tribology Int 2009; 42: 243–249.

11.

Hengjun Liu Yanan Pei Dong Xie . Surface Modification of Ultra-high Molecular Weight Polyethylene (UHMWPE) by Argon Plasma. Appl Surf Sci 2010; 256: 3941–3945.

12.

Redlich

Fortuniak

. Sposób wykorzystania odpadów balistycznych wyrobów polietylenowych. Tworzywa Sztuczne i Chemia 2010; 3: 8–8.

13.

Kanesalingam

Lyndon

Rajiv

. Application of Wool in High-velocity Ballistic Protective Fabrics Textile. Text Res J 2010; 80: 1083–1083.

14.

Fejdyś

Łandwijt

Struszczyk

. Effect of Accelerated Ageing Conditions on the Degradation Process of Dyneema® Polyethylene Composites. Fibres Text East Eur 2011; 84: 60–65.