Interfacial Integrity in Robotic Over-Printing: Joining Continuous Fiber Composites to Prefabricated Liners

A Technical Review of Hybrid Manufacturing Challenges at the Composite-Liner Interface

1. The Interface Problem: Thermal Incompatibility at the Bonding Frontier

1.1 Fundamental Challenge

The deposition of molten continuous fiber thermoplastic tape onto a cold polymer substrate creates a thermal shock scenario that fundamentally challenges interfacial integrity. In thermoplastic tape winding (TTW) and automated fiber placement (AFP), the incoming prepreg tape must reach temperatures sufficient for matrix flow and molecular interdiffusion—typically 280-400°C for engineering thermoplastics like PEEK, PEKK, or carbon fiber-reinforced polyamide. Meanwhile, the prefabricated liner, whether high-density polyethylene (HDPE) or polyamide (PA6/PA11), maintains ambient or moderately elevated temperatures.

This temperature differential creates several competing failure mechanisms:

Thermal Gradient at the Nip Point During Over-Printing

Cross-sectional view showing temperature distribution during thermoplastic tape deposition onto a polymer liner. Illustrates the steep thermal gradient from molten tape (~350°C) through the interface to the cool liner substrate (~25-80°C), with heat flux vectors and consolidation roller position.

1.2 The Bonding Mechanism: Intimate Contact and Autohesion

Successful fusion bonding between the incoming tape and substrate requires satisfaction of two sequential conditions:

The degree of healing D_h follows: D_h = (t/t_w)^1/4, where t is contact time and t_w is the reptation time (temperature-dependent).

For dissimilar materials (e.g., CF/PEEK tape onto PA6 liner), true autohesion is impossible; bonding instead relies on mechanical interlocking, van der Waals interactions, and potential chemical bonding at functionalized surfaces.

1.3 Failure Statistics in COPV Applications

The criticality of the liner-composite interface is underscored by failure statistics from composite overwrapped pressure vessel applications:

• Liner failure is the second leading cause of COPV failure after composite damage
• Interface debonding contributes to liner collapse during rapid depressurization cycles
• The 2016 SpaceX Falcon 9 pad explosion was attributed to failure of a COPV, where frozen solid oxygen accumulated between the aluminum liner and composite overwrap, demonstrating the catastrophic consequences of interface defects

2. Surface Activation Technologies

The inherently low surface energy of polyolefins (HDPE: 28-30 mN/m) and the semi-crystalline nature of engineering polymers create barriers to adhesive bonding. Surface activation technologies aim to increase surface energy, introduce polar functional groups, and create mechanical interlocking features.

2.1 Atmospheric Plasma Treatment

Plasma treatment represents the most widely adopted surface activation technology for thermoplastic liner materials, offering non-contact processing and integration into continuous manufacturing lines.

Mechanism:

• Interaction of plasma species (charged particles, molecular radicals, excited species) with the polymer surface
• Introduction of oxygen-containing functional groups (C-O, C=O, O-C=O, O-H)
• Surface oxidation increases surface energy from ~28 mN/m to >50 mN/m

Quantitative Results:

2.2 Mechanical Abrasion: Grit Blasting and Sanding

Mechanical surface preparation creates topographical features for mechanical interlocking and removes surface contaminants.

Grit Blasting Results (CFRP Adherends):

• Peel-ply + grit blasting: Highest average roughness (7.35 μm), highest surface energy (49.1 mJ/m²)
• Manual sanding: Effective but operator-dependent
• Random sanding direction: Highest shear strength reported

2.3 Laser Surface Treatment

Laser ablation offers precise, non-contact surface modification with the ability to create controlled micro-scale features.

Nanosecond Pulsed Laser Treatment for PA11-CFRP Interfaces:

A breakthrough study demonstrated the effectiveness of nanosecond pulsed laser treatment for enhancing bonding between PA11 liners and CFRP in Type IV hydrogen storage vessels:

Mechanism: Formation of periodic grooves on the PA11 surface increases roughness and mechanical interlocking area. UV wavelengths (355 nm) additionally introduce polar functional groups (O-C=O, O-O) through photo-chemical bond breaking rather than purely thermal ablation.

2.4 Chemical Primers and Coupling Agents

Silane coupling agents provide chemical bridging between dissimilar materials:

Mechanism:

• Hydrolyzable groups (typically alkoxy: -OCH₃, -OC₂H₅) react with hydroxyl groups on substrates
• Organic functional groups (amino, mercapto, epoxy) react with polymer matrices
• Creates covalent "adhesive bridge" at the interface

Performance Data:

• Plasma-treated CF thermoplastic + mercapto silane monolayer: 27 MPa shear stress (versus 10 MPa untreated, 22 MPa plasma-only)
• Dipodal silanes: Up to 10⁵× greater hydrolysis resistance for primer applications
• Aminosilane coupling agents: Achieved 1.53 MPa adhesive strength in thermal protection coatings at 13 wt% loading

2.5 Comparative Surface Treatment Performance

Lap Shear Strength by Surface Treatment Method

Note: Cohesive failure in the substrate or adhesive indicates the interface is no longer the weakest link—the desired outcome.

3. Thermal Bonding Windows

3.1 Critical Temperature Thresholds

Successful fusion bonding requires maintaining the interface within a narrow "bonding window" defined by the thermal properties of both materials:

Thermal Properties of Common Liner Materials

Composite Tape Processing Temperatures:

3.2 Thermal Management Strategies

Liner Pre-Heating:

Elevating liner temperature reduces the thermal gradient and shock severity while promoting better wetting:

• HDPE liners: Pre-heat to 60-80°C (below T_m with safety margin)
• PA liners: Pre-heat to 80-120°C
• Method: Infrared heaters, hot air circulation, heated mandrels

Layup Speed Reduction:

Slower deposition allows more time for heat conduction into the liner, enabling surface temperatures to equilibrate:

• Trade-off: Reduced productivity
• Typical adjustment: 50-70% speed for first ply versus steady-state

Laser/Heat Source Power Modulation:

Closed-loop temperature control using pyrometry enables real-time adjustment:

• Hoop versus axial layups require different power settings due to varying dwell times
• First-ply deposition onto dissimilar liner requires reduced power to prevent burn-through

3.3 Liner Compatibility Requirements

For direct fusion bonding (without intermediate adhesive), the liner and tape matrix should ideally share the same base polymer to enable true autohesion:

• CF/PA6 tape → PA6 liner: Compatible (molecular interdiffusion possible)
• CF/PEEK tape → HDPE liner: Incompatible (no interdiffusion; relies on mechanical interlocking and surface functionalization)

When material systems are incompatible, surface activation becomes critical, and bond strength expectations must be adjusted accordingly.

4. Residual Stress and Warpage: CTE Mismatch Effects

4.1 The CTE Mismatch Problem

The coefficient of thermal expansion mismatch between carbon fiber composite overwrap and polymer liner materials represents a fundamental source of residual stress accumulation.

Coefficient of Thermal Expansion Comparison

4.2 Residual Stress Mechanisms

Residual stresses develop during cooling from processing temperature to ambient conditions:

4.3 Consequences for Pressure Vessel Applications

Liner Collapse Mechanism:

During rapid depressurization of Type IV hydrogen vessels, the composite overwrap (with lower CTE and higher stiffness) constrains the liner's thermal contraction less than its pressure-induced contraction. This creates a pressure differential at the liner-CFRP interface:

• High internal pressure expands liner into tight contact with composite
• Rapid depressurization: Internal pressure drops faster than liner can contract
• Liner buckles inward into the gap between its equilibrium position and the composite

Contributing Factors:

• Higher maximum hydrogen pressure → greater pressure differential
• Faster depressurization rate → less time for equilibration
• Lower residual pressure → larger pressure difference
• Weak interfacial bonding → easier debonding initiation

4.4 Mitigation Strategies

5. Case Study Analysis

5.1 Hydrogen Composite Overwrapped Pressure Vessels (COPVs)

Application Context: Type IV and emerging Type V COPVs are critical enablers for hydrogen mobility, requiring storage at 350-700 bar with minimal weight penalty. The interface between polymer liner and carbon fiber composite overwrap directly impacts:

• Structural integrity under cyclic pressurization
• Hydrogen permeability and leakage rates
• Resistance to liner collapse during rapid refueling/discharge

Liner Material Selection:

Property	HDPE	PA6	PA11	Best for H₂
H₂ Permeability	Highest	Lower	Lower	PA6
Cost	Lowest	Medium	Higher	HDPE
CTE Match	Poor	Fair	Fair	PA6
Moisture Sensitivity	None	High	Low	HDPE, PA11

Hydrogen Permeability Data:

At 288 K and 70 MPa:

• PA6 exhibits the strongest hydrogen permeation resistance
• PA11 has approximately 12.5% higher diffusion coefficient than PA6
• HDPE has approximately 350% higher diffusion coefficient than PA6

The GTR 13 international standard specifies maximum allowable hydrogen permeation rate of 46 Ncm³·h⁻¹·L⁻¹ at 1.15× nominal working pressure and 55°C.

Figure 6: Type IV COPV Cross-Section and Interface Detail

Cross-sectional view of Type IV COPV showing (a) overall vessel construction with polymer liner, composite overwrap, and metallic bosses, and (b) detail of liner-composite interface with surface treatment features.

5.2 Automotive Structural Reinforcement: B-Pillar Over-Molding

Application Context: Automotive B-pillars (center pillars) are critical structural members for side-impact protection. Hybrid manufacturing approaches combine steel or aluminum stampings with composite reinforcements to achieve weight reduction while meeting crashworthiness requirements.

Hybrid Molding Approach:

• Organosheet (e.g., 47 vol-% glass fabric/PA6) is compression-molded as carrier
• Short-fiber reinforced polymer (e.g., 30 wt-% glass/PA6/6) is injection over-molded for ribs
• Expanding adhesive (epoxy-based foam) bonds composite to metal structure

Interface Considerations:

• PA6 carrier + PA6 overmold: Same-polymer system enables fusion bonding
• Composite-metal interface: Requires adhesive bonding with surface preparation
• Expanding adhesive: Accommodates tolerance variations and fills gaps

Figure 7: Automotive B-Pillar Hybrid Structure

Exploded view of hybrid B-pillar construction showing steel outer panel, composite reinforcement insert (compression-molded organosheet + injection over-molded ribs), and adhesive interface layer.

5.3 Comparative Failure Mode Analysis

Understanding whether failures occur at the interface (adhesive failure) or within the substrate (cohesive failure) provides critical feedback on surface treatment effectiveness:

Failure Mode Analysis by Surface Treatment

6. Future Directions and Research Gaps

6.1 Identified Research Gaps

6.2 Emerging Technologies

7. Conclusions

The interface between continuous fiber composite overwrap and prefabricated polymer liner represents the critical weak link in hybrid manufacturing systems for pressure vessels and structural reinforcements. This review has established:

References

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