Description:The present invention relates to the field of high-pressure composite cylinders, particularly Type IV composite overwrapped pressure vessels (COPVs). More specifically, the invention pertains to an external protection layer made of glass fiber composite wound over a carbon fiber composite shell to enhance durability, thermal resistance, and impact protection of the composite cylinder.

4. Background of the Invention / Prior Art:
Composite overwrapped pressure vessels (COPVs) are widely used for storing compressed gases such as CNG and hydrogen, especially in automotive and aerospace sectors. Type IV COPVs typically consist of a polymeric liner overwrapped with continuous fiber-reinforced composite materials, most commonly carbon fiber reinforced polymers (CFRPs).
CFRPs are preferred due to their high strength-to-weight ratio and fatigue resistance. However, they are brittle and sensitive to surface damage from external impact or high temperatures. This limits their durability, especially when exposed to harsh environments or mishandling.

Prior Arts:
1. Patent US20060205077A1 discusses COPVs with multiple fiber layers but lacks focus on external protection against UV and thermal exposure.
2. Patent CN104113602A proposes a composite cylinder structure but does not integrate a separate GFRP layer specifically for thermal and UV resistance.
3. Patent WO2015055081A1 discloses a multi-layered vessel structure, but its configuration focuses more on structural efficiency than on external protection or insulation.
4. Thus, the prior art lacks a practical solution where glass fiber is selectively used as a top winding layer for protection, insulation, and durability enhancement while the inner CFRP carries the mechanical load.

5. Summary of the Invention:
This invention introduces a multi-layer composite vessel construction comprising a CFRP structural layer and a GFRP protective layer. The carbon fiber layer is responsible for bearing the high internal pressures, while the glass fiber layer, wound externally, serves to:
• Protect the underlying CFRP from impact damage
• Shield the structure from UV radiation
• Provide thermal insulation due to glass fiber's low thermal conductivity
• Enhance overall durability and performance in extreme environmental conditions
The glass fiber winding covers the entire cylindrical body and dome ends, ensuring comprehensive protection during the service life of the COPV.

6. Description of Drawings
Figure 1: Cross-section of a COPV showing liner, CFRP winding, and external GFRP layer
Figure 2: Schematic of filament winding setup applying GFRP over CFRP
Figure 3: Stress distribution diagram showing impact absorption in GFRP layer

7. Detailed Description of the Invention
7.1 Overview of the Composite Cylinder Structure
The invention pertains to an improved high-pressure composite cylinder, commonly referred to as a Type IV Composite Overwrapped Pressure Vessel (COPV). This type of vessel is widely used for applications involving compressed gases, such as CNG (Compressed Natural Gas) and hydrogen fuel storage, especially in mobile and stationary systems. The vessel typically comprises a blow-molded polymeric liner, which is non-load-bearing, surrounded by a composite overwrap that provides the structural integrity to withstand internal pressure.
In conventional systems, this overwrap is made from carbon fiber reinforced polymer (CFRP) owing to its exceptional strength-to-weight ratio and durability. However, while CFRP offers excellent mechanical performance, it remains susceptible to environmental degradation, including UV radiation, thermal expansion and contraction, and external impacts. The proposed invention mitigates these weaknesses through the integration of an outer glass fiber reinforced polymer (GFRP) protective layer, applied by filament winding over the cured CFRP layer.

7.2 Construction Sequence and Material Configuration
The proposed structure is a multi-layered composite configuration, comprising the following key components:
1. Inner Polymeric Liner:
Typically constructed from high-density polyethylene (HDPE), polyamide (nylon), or polyethylene terephthalate (PET), this layer forms the leak-proof inner wall of the vessel. It provides containment of the stored fluid but bears no structural load.
2. Primary CFRP Layer:
This layer is the main load-bearing component and is wound in helical and hoop orientations using epoxy-impregnated carbon fibers. It is responsible for sustaining the high tensile hoop and axial stresses generated due to internal pressurization.
3. Secondary GFRP Layer (Protective Layer):
Once the CFRP layer is cured or semi-cured, an additional winding of epoxy-impregnated E-glass or S-glass fiber is applied uniformly across the entire exterior of the cylinder, including the cylindrical section and both dome ends.
The curing process is typically carried out after the entire winding is completed, under temperature-controlled conditions (e.g., 110–130°C) to ensure complete polymer cross-linking and strong interfacial bonding between CFRP and GFRP.

7.3 Functional Benefits of the GFRP Layer
The application of an external GFRP layer introduces multiple functional benefits that significantly enhance the utility and reliability of the vessel, including:
7.3.1 Impact Resistance
CFRP, despite its high stiffness and strength, has poor impact tolerance due to its brittle fracture behavior. GFRP, on the other hand, offers superior energy absorption and crack-arresting capabilities. The GFRP layer acts as a mechanical buffer, distributing impact forces and protecting the underlying carbon structure from surface cracks, delamination, or fiber breakage. This is especially crucial in automotive environments where tanks are vulnerable to accidental drops, tool contact, or debris.
7.3.2 UV Protection
Carbon fiber epoxy matrices are known to degrade upon prolonged exposure to ultraviolet (UV) radiation, leading to loss of mechanical integrity. Glass fiber composites, in contrast, exhibit higher UV resistance. The GFRP layer acts as a UV shielding layer, preventing sunlight and UV rays from penetrating to the CFRP core, thereby enhancing the lifespan of the cylinder when used in outdoor conditions.
7.3.3 Thermal Insulation
Carbon fibers are conductive to heat, with thermal conductivity ranging from 20–200 W/m·K. In contrast, glass fibers possess low thermal conductivity (~1 W/m·K). When wrapped as an external shell, the GFRP layer acts as a thermal insulator, protecting the cylinder from rapid external temperature changes and helping to maintain a stable internal environment for the gas. This is particularly beneficial in climates with extreme ambient temperature variations.
7.3.4 Fire and Heat Resistance
GFRP layers can also be treated with flame retardant epoxy systems or include ceramic-based fillers to improve fire resistance. This reduces the likelihood of structural failure under external heat or fire scenarios, enhancing safety in critical use cases such as firefighting vehicles or industrial fuel systems.

7.4 Filament Winding Technique and Orientation
The application of both CFRP and GFRP layers utilizes precision-controlled filament winding equipment, which ensures:
• Consistent fiber tension
• Controlled resin impregnation
• Layer thickness uniformity
• Reproducible winding patterns
The CFRP winding is typically composed of:
• Hoop winding (for hoop stress containment)
• Helical winding (for axial stress support)
Once the CFRP winding is completed, the GFRP protective winding is applied in similar helical and hoop orientations. However, the winding angle and fiber density may vary to focus on:
• Edge protection at shoulder regions
• Increased layers over high-stress dome regions
• Thinner coverage on mid-cylinder for weight optimization
The fiber-matrix ratio is maintained at approximately 65:35 to 70:30 to ensure both mechanical performance and thermal stability.

7.5 Layer Integration and Curing Process
After both CFRP and GFRP windings are completed, the vessel undergoes a controlled curing cycle inside an autoclave or heated oven. The curing process parameters typically include:
• Temperature: 110°C to 130°C
• Time: 4 to 6 hours
• Pressure (if autoclaved): 3 to 5 bar internal/external
The dual-material layup ensures excellent interlaminar bonding due to the compatibility of resin systems. A co-curing technique may also be used to reduce cycle times and improve adhesion at the GFRP-CFRP interface.
Optional post-curing surface treatments such as clear UV-stable polyurethane coating or embedded wear indicator layers may be added.

7.6 Structural Testing and Performance Validation
Prototypes developed using the proposed hybrid winding structure undergo a series of performance and safety tests to ensure compliance with ISO 11119, ISO 11439, or BIS standards:
7.6.1 Drop and Impact Tests
• Cylinders are dropped from 1.8 meters onto hard surfaces.
• The GFRP-protected cylinders exhibit minimal surface chipping and no CFRP fiber exposure.
7.6.2 Heat Resistance Tests
• Exposed to 100°C–120°C external temperatures for 4 hours.
• Internal temperatures remain 30–40% lower due to thermal insulation of GFRP.
7.6.3 UV Aging Tests
• Exposed to UV light for 1000 hours.
• GFRP-covered cylinders retain surface gloss and color, indicating minimal degradation.
7.6.4 Burst and Pressure Cycling
• GFRP layer does not interfere with CFRP’s burst performance.
• It supports the overall structural integrity during rapid pressure cycles (e.g., 10,000 cycles from 0 to 300 bar).

7.7 Optional Enhancements and Adaptations
The system is highly customizable. Depending on the application (automotive, aerospace, storage), the following enhancements can be incorporated:
• Colored GFRP layers for identification or branding.
• Nano-enhanced resins in GFRP layer to improve heat resistance.
• Embedded sensor foils between GFRP and CFRP for smart monitoring of pressure, temperature, or external impacts.
• Modular layer thickness design for different parts of the cylinder.

7.8 Manufacturing Efficiency and Cost Optimization
Although glass fiber is cheaper than carbon fiber, its strategic placement only on the outer surface ensures cost-effective performance enhancement. The total additional cost of the GFRP layer is offset by:
• Extended cylinder lifespan
• Reduced need for protective casings
• Lower rejection rates from damage
• Enhanced consumer safety
This structure aligns with Make in India objectives, enabling scalable local production of COPVs for CNG and hydrogen vehicle adoption.

7.9 Industrial and Commercial Applications
The described technology can be implemented across a wide range of industries, including:
• Automotive (CNG and hydrogen-fueled vehicles)
• Aerospace (satellite and rocket propellant storage)
• Defense (high-pressure air cylinders, oxygen tanks)
• Industrial gas storage and transport systems
• Renewable energy (hydrogen fuel storage for solar-hydrogen integration)
The modular winding design can be adapted for various cylinder sizes ranging from 2 liters to over 300 liters capacity.
, Claims:1. A composite overwrapped pressure vessel comprising:
o a polymeric liner,
o a load-bearing carbon fiber reinforced polymer (CFRP) layer wound over the liner, and
o an external glass fiber reinforced polymer (GFRP) layer wound over the CFRP layer for protection and insulation.
2. The vessel of claim 1, wherein the GFRP layer extends continuously across the cylindrical body and both dome-shaped ends.
3. The vessel of claim 1, wherein the GFRP layer provides impact resistance to protect the CFRP layer from mechanical damage.
4. The vessel of claim 1, wherein the GFRP layer exhibits low thermal conductivity and acts as a thermal insulator.
5. The vessel of claim 1, wherein the GFRP layer is resistant to ultraviolet (UV) radiation and environmental degradation.
6. The vessel of claim 1, wherein the thickness of the GFRP layer ranges from 1.5 mm to 4 mm.
7. The vessel of claim 1, wherein the GFRP winding includes helical and hoop winding angles customized to the vessel geometry.
8. The vessel of claim 1, wherein the GFRP layer is embedded with visual indicators for UV degradation monitoring.
9. The vessel of claim 1, wherein the glass fiber used comprises E-glass or S-glass type materials.
10. A method of manufacturing the vessel of claim 1, comprising steps of:
• forming a polymeric liner,
• winding CFRP layer using filament winding,
• overwrapping the CFRP with GFRP,
• and curing the composite assembly.