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Pultruded Profile Without Peelply: Lower Cost, Full Performance for Wind Turbine Blades
1. What Is a Pultruded Profile Without Peelply?
A pultruded profile without peelply is a continuous glass fiber reinforced polymer (GFRP) structural section — typically a flat plate or spar cap strip — manufactured by the pultrusion process in which the conventional peel-ply surface layer has been eliminated through advanced resin chemistry and process optimization.
In traditional pultruded composite profiles destined for secondary bonding (such as wind turbine blade spar caps), a woven fabric layer called the "peel ply" is co-cured onto the surface during pultrusion and then physically torn off immediately before structural adhesive is applied. This creates a chemically clean, micro-textured surface ready for bonding. In the peel-ply-free design, the surface preparation function is built directly into the resin system, making the separate peel-ply layer unnecessary while maintaining or improving adhesion performance and reducing overall material cost.
Zhejiang Zhenshi New Material Co., Ltd., headquartered at No. 1 Guangyun South Road, Tongxiang Economic Development Zone, Zhejiang Province, is a leading polymer composite materials manufacturer whose Pultruded Profile Without Peelply targets the demanding specifications of onshore wind turbine blades in the 70–90 m range, with a modulus concentrated in the 60–65 GPa band.
2. The Pultrusion Process: A Step-by-Step Technical Overview
Pultrusion is a continuous manufacturing process for constant-cross-section fiber reinforced polymer (FRP) profiles. Unlike filament winding or resin transfer moulding, it operates in a linear, pull-through mode that produces structural sections at high output rates with excellent longitudinal property consistency.
2.1 Process Stages
Stage 1 — Fibre delivery: Continuous glass fiber rovings are fed from a creel rack, passing through a tensioning and guide system that controls fibre straightness and spacing. For Zhenshi's pultruded profile without peelply, the primary reinforcement is high-quality E-glass rovings selected for consistent tex weight and sized for optimum resin wet-out.
Stage 2 — Resin impregnation: Fibres are pulled through an open resin bath or injected resin system. In the peelply-free design, the resin bath contains the chemically modified formulation that delivers both the structural matrix properties and the bonding-surface chemistry described in Section 4.
Stage 3 — Preforming: Wet fibres pass through guide plates that progressively consolidate and align them into the target cross-section geometry — typically a flat plate 50–200 mm wide, 2–8 mm thick for blade spar cap applications.
Stage 4 — Heated die and curing: The consolidated stack enters a precision steel die, maintained at controlled temperature zones (typically 140–180°C). The polymerisation exotherm and die temperature combine to fully cure the matrix. This is the stage where surface chemistry is activated.
Stage 5 — Pulling and cutting: Gripper-type haul-off units apply the pulling force that draws material continuously through the process. Cut-off saws section the profile to the required length.
3. The Role of Peel Ply in Traditional Composites
In the traditional composite fabrication workflow, a peel ply is a woven fabric (typically nylon or polyester) applied to the surface of a composite part during cure. Its function is purely preparatory: when it is peeled away just before secondary bonding, it leaves behind a micro-textured, chemically clean surface that promotes excellent adhesive bonding without requiring mechanical abrasion, solvent wiping, or grit blasting.
For pultruded spar cap profiles used in wind turbine blades, the peel ply serves as the interface layer between the pultruded strip and the infused blade shell laminate or structural adhesive film. Without adequate surface preparation, the bond between spar cap and blade skin can fail at loads far below the structural design limit — a critical safety concern in blades experiencing hundreds of millions of fatigue load cycles over a 20–25 year design life.
Despite its utility, the peel ply adds cost, weight, and a mandatory manual processing step. It must be stored, transported, applied during the pultrusion run, and then removed on-site — with strict controls to ensure no contamination from the peel ply fabric itself (release agent residues, fibre fragments) reaches the bonding surface.
4. How the Peelply Is Eliminated: Resin Formulation & Chemical Modification
The technical innovation enabling the peelply-free profile is an adjusted resin formulation combined with surface chemical modification. Zhenshi engineers the resin system to fulfil two simultaneous roles that were previously separated across the matrix resin and the peel-ply layer:
4.1 Structural Matrix Function
The resin must provide full mechanical performance — sufficient crosslink density for Tg ≥ 105°C, matrix-dominated transverse strength ≥50 MPa, and interlaminar shear strength ≥60 MPa (all per published Zhenshi data). These requirements constrain the resin chemistry and limit the modifications available for surface treatment.
4.2 Surface Chemistry Activation
By incorporating reactive surface-active agents into the resin formulation, the cured profile surface develops a micro-scale chemical reactivity and mechanical texture that promotes adhesive bonding without the need for peel-ply removal. The surface chemistry is typically based on polar functional groups that interact with common epoxy structural adhesives — the same adhesives used in blade assembly — through primary chemical bonding rather than purely mechanical interlocking.
4.3 Die Surface Engineering
The heated die surface finish is co-optimized with the resin chemistry. Rather than the smooth mirror-finish die used for structural appearance profiles, the peelply-free die incorporates a controlled micro-texture that is imparted to the profile surface at the moment of demoulding — analogous to the roughness left by peel-ply removal but achieved deterministically through die geometry rather than fabric weave pattern.
Engineering significance: Eliminating the peel-ply does not mean eliminating surface preparation — it means integrating surface preparation into the manufacturing process itself. The resulting surface is always in the correct state for bonding, with no risk of contamination from peel-ply residues or operator error during the peel-removal step.
5. Fibre Volume Fraction and Why 67–74% Matters
The fibre volume fraction (Vf) is the single most important microstructural parameter controlling the mechanical properties of a pultruded profile. Zhenshi's peelply-free profile achieves a Vf of 67–74% — exceptionally high for a glass fibre pultruded product, and a direct result of the high tension and precision fibre guidance maintained through the pultrusion process.
5.1 Vf and Longitudinal Modulus
By the rule of mixtures, the 0° tensile modulus of a unidirectional composite scales almost linearly with Vf: E₁ ≈ Vf × Ef + (1 - Vf) × Em. For E-glass (Ef ≈ 72–76 GPa) in an epoxy matrix (Em ≈ 3.5 GPa), a Vf of 67% gives E₁ ≈ 50.5 GPa; at Vf = 74%, E₁ ≈ 56.5 GPa. The published 60–65 GPa target reflects the additional contribution of the slightly higher modulus rovings selected for the E8-390 and premium grades.
5.2 Vf and Manufacturing Stability
Very high Vf (above ~70%) in pultrusion introduces process challenges: resin-starved regions, void formation, and increased die pressure. Zhenshi's process control — including optimised resin viscosity, die temperature profiles, and pulling speed — maintains void content typically below 1% at the target Vf range, which is essential for fatigue performance in cyclic-load wind blade applications.
5.3 Density Implications
At Vf = 67–74% with E-glass (density 2.54 g/cm³) and epoxy matrix (density ~1.2 g/cm³), the composite density works out to approximately 2.1–2.2 g/cm³ — precisely the range published in Zhenshi's data sheet. This is significantly lower than steel (7.85 g/cm³) and aluminium (2.70 g/cm³), enabling large structural sections to remain weight-competitive.
6. Mechanical Performance Data — Grades E7, E8, and E8-390
Zhenshi publishes three grade variants of the pultruded profile without peelply, reflecting increasing performance tiers corresponding to higher-specification roving grades and tighter process windows. The full property dataset is reproduced below for engineering reference.
| Property | E7 Grade | E8-390 Grade | Premium Grade |
|---|---|---|---|
| Fibre Volume Fraction (%) | 69–73 | 69–72 | 72–74 |
| 0° Tensile Strength (MPa) Char. | ≥1,260 | ≥1,300 | ≥1,400 |
| 0° Tensile Modulus (GPa) Avg. | 62–63 | 63–64 | ≥65 |
| 0° Compression Strength (MPa) Char. | ≥1,100 | ≥1,200 | ≥1,200 |
| 0° Compression Modulus (GPa) | 62–63 | 63–64 | ≥65 |
| 90° Tensile Strength (MPa) Char. | ≥50 | ≥50 | ≥52 |
| 90° Tensile Modulus (GPa) | ≥15 | ≥15 | ≥16 |
| 90° Compression Strength (MPa) Char. | ≥155 | ≥150 | ≥160 |
| 90° Compression Modulus (GPa) | ≥19 | ≥19 | ≥20 |
| In-Plane Shear Strength (MPa) | ≥60 | ≥60 | ≥65 |
| In-Plane Shear Modulus (GPa) | ≥5.0 | ≥5.0 | ≥5.2 |
| Interlaminar Shear Strength (MPa) | ≥60 | ≥60 | ≥65 |
| Flexural Strength (MPa) | ≥1,200 | ≥1,200 | ≥1,200 |
| Flexural Modulus (GPa) | ≥55 | ≥50 | ≥55 |
| Glass Transition Temp. Tg (°C) | ≥105 | ≥105 | ≥105 |
| Fatigue m (R=0.1) | ≥8.5 | ≥8.5 | ≥8.5 |
| Density (g/cm³) | 2.1–2.2 (all grades) | ||
| Width (mm) | 50–200 (all grades) | ||
| Thickness (mm) | 2–8 (all grades) | ||
Note: Characteristic values are statistical lower bounds per standard test methods (EN ISO 527, EN ISO 14126 etc.). Actual production values typically exceed these minima. Data source: Zhenshi product page.
7. Fatigue Performance and Long-Term Durability
Wind turbine blades experience complex fatigue loading throughout their operational life. A typical onshore 70–90 m blade undergoes in the order of 10⁸–10⁹ fatigue cycles over a 20–25 year design service life, with load amplitudes driven by wind gusts, turbulence, gravity cycling, and emergency stops.
7.1 The Fatigue m Parameter
The published fatigue parameter for Zhenshi's peelply-free profile is m ≥ 8.5 (at R = 0.1). The parameter m characterises the slope of the S-N curve in the Wöhler diagram representation log(σ) vs log(N). A higher m value means the material retains more of its static strength at high cycle counts — a critical advantage in wind blade spar caps where blade mass constraints prevent conservative over-sizing.
7.2 Interlaminar Shear Strength and Delamination Resistance
The interlaminar shear strength (ILSS) of ≥60–65 MPa is particularly relevant to blade performance. Delamination within the pultruded spar cap is one of the primary damage modes observed in fatigue testing of blade prototypes. The high Vf and controlled void content of Zhenshi's profiles are the primary contributors to this ILSS level.
7.3 Environmental Durability
Wind turbine blades operate in humid, salt-laden, thermally cycling environments. Zhenshi's pultruded profile without peelply inherits the key durability characteristics of glass fibre pultruded composites: water resistance, corrosion resistance, and environmental stability across the operating temperature range expected at onshore wind farm sites. The Tg ≥ 105°C ensures dimensional stability in high-temperature microenvironments inside the blade shell.
8. Cost Economics: Where the Savings Come From
The "without peelply" designation is not merely a process simplification — it is a deliberate cost engineering decision that reduces total spar cap material cost through several independent pathways:
Industry context: For a 90 m blade with a total pultruded spar cap mass of 2,000–3,500 kg, even a 5–8% reduction in spar cap material unit cost translates to several thousand dollars per blade set — significant at the scale of a 250 MW wind farm.
9. Integration in 70–90 m Onshore Wind Turbine Blades
The primary application for Zhenshi's pultruded profile without peelply is the spar cap (also called main spar flange or load-carrying laminate) of onshore wind turbine blades in the 70–90 m length range — corresponding to turbine rated power of 3–6 MW, the dominant segment of current onshore wind energy installations.
9.1 Spar Cap Function and Loading
The spar cap is the primary structural element of the blade, running along the blade span on both the suction and pressure sides of the aerofoil. It resists the dominant flapwise bending moment — the load direction responsible for the highest stress levels in the blade laminate. The pultruded profile's outstanding 0° tensile and compressive modulus (62–65 GPa) and strength directly determine the blade's stiffness, tip deflection, and resistance to buckling.
9.2 Why 60–65 GPa Is the Target Modulus Range
For blades in the 70–90 m class, structural analysis and aeroelastic modelling define an optimal spar cap stiffness band. Too low a modulus requires more material mass to meet tip deflection limits; too high a modulus (full carbon at ≥140 GPa) is cost-prohibitive at this blade size when glass fibre can meet the stiffness requirement with a modest increase in spar cap thickness. The 60–65 GPa glass pultruded profile sits in the "sweet spot" — sufficient stiffness without carbon-fibre cost.
9.3 Width and Thickness Range
Zhenshi supplies profiles in widths 50–200 mm and thicknesses 2–8 mm. Multiple strips are typically laid side-by-side and stacked (3–8 layers) to build the spar cap to its structural design thickness, with structural adhesive or infused resin between layers for interlaminar bonding. The peelply-free surface chemistry supports this multi-layer bonding approach.