GLS High-Stretch Series: Covestro TPU, Mechanical Strength

Why Covestro Desmopan for PPF Base Film

The single largest variable in PPF performance is the quality of the TPU base polymer. Two films with identical nominal thickness can have radically different mechanical behaviour in the field — different elongation at break, different stress response underimpact, different long-term UV stability — purely because of differences in the underlying TPU chemistry and supply chain operations consistency.

Covestro (formerly Bayer MaterialScience) produces Desmopan, an aliphatic Thermoplastic Polyurethane that has become the de facto standard for premium PPF base film. The key reasons:

• Consistent molecular weight distribution. Covestro's continuous polymerization process produces TPU pellets with very narrow molecular weight distribution, which translates directly to consistent film caliper and uniform mechanical properties across the roll width and across production lots.
• Aliphatic isocyanate backbone. The hard segment of Desmokan uses aliphatic diisocyanates (specifically H12MDI — hydrogenated methylene diphenyl diisocyanate). This eliminates the aromatic ring structures that are the primary chromophore in UV-initiated polymer degradation — which is why aliphatic TPU does not yellow and aromatic TPU does.
• Hydrolytic stability. Desmokan has better resistance to moisture hydrolysis than polyester-based TPU alternatives, which matters for films installed in high-humidity environments (coastal cities, monsoon climates).
• Thermal processing window. Covestro specifies the melt processing window for each grade, allowing film manufacturers to optimize extrusion parameters without thermal degradation of the polymer.

GLS150 vs GLS175: Specification Comparison

Both GLS150 and GLS175 share the same Covestro Desmokan base polymer and the same Ashland PSX adhesive system. The difference is nominal caliper — and caliper matters for both mechanical protection performance and installation ergonomics.

The additional 25μm in GLS175 provides measurably higher tensile strength and better resistance to stone chip impact — the kinetic energy absorption of a 175μm film is approximately 17% higher than a 150μm film at equal strain rates. However, GLS175 requires slightly more heat energy to reach full activation temperature during installation, and its greater stiffness reduces drape conformability on very tight radii. GLS150 is the preferred product for complex geometries with deep draws; GLS175 is preferred for flat panels where maximum stone chip protection is the priority.

What the 319.47% Elongation Figure Actually Means

Elongation at break is the most discussed — and most misunderstood — mechanical specification in PPF. A figure of 319.47% sounds like a guarantee of supreme ductility, and in one sense it is. But what does it mean in practice?

When a PPF film is subjected to a tensile test (ASTM D882), a strip of film is gripped at both ends and stretched at a constant rate of strain until it breaks. The elongation at break is the percentage increase in length at the point of rupture compared to the original gauge length. For a 50mm gauge length specimen, 319.47% elongation means the film stretched to 209.7mm before failing.

What this tells you about real-world performance:

• Stone chip resistance: The high elongation capacity means the film can absorb impact energy by deforming plastically rather than cracking. A stone hitting the film at 120 km/h will cause the film to stretch and absorb energy rather than fracture.
• Conformability: The high elongation window directly translates to how much the film can be stretched during heat-activated installation without risk of tearing at the anchor points.
• Recovery behaviour: High-quality aliphatic TPU with this elongation profile also exhibits good elastic recovery — meaning the film does not permanently deform under the stress of installation stretch. It returns to its nominal dimensions after post-heat cure.

Gloss Stability Under GCC Climate Stress

GCC markets (Saudi Arabia, UAE, Qatar, Kuwait, Oman, Bahrain) present one of the world's most demanding UV and thermal environments for polymeric surface films. A dark-coloured vehicle in direct Saudi summer sunlight can experience surface temperatures of 85–95°C on horizontal panels. The PPF on those panels is subject to both thermal oxidation (accelerated by heat) and UV photodegradation simultaneously.

The Wansen GLS series topcoat formulation is specifically designed for this combined stress profile. Key design decisions:

• UVA + UVB absorber package. The ceramic topcoat incorporates a stabiliser package that absorbs both UVA (320–400nm) and UVB (280–320nm) radiation before it reaches the TPU layer beneath. This extends the functional life of the aliphatic TPU by preventing photo-oxidation chain reactions.
• Thermal antioxidant addition. Heat stabilisers are added to the TPU base to prevent thermal oxidation at sustained temperatures up to 120°C (short-term spikes).
• Crosslink density in topcoat. The topcoat has a higher crosslink density than the base film, providing a barrier layer that resists scratching and surface marring — the primary gloss-loss mechanisms in PPF.

After 1000 hours of accelerated UV exposure per ASTM G154 (using UV-A 340nm lamps), Wansen's GLS series films retain more than 92% of their initial 60° gloss reading. This is a critical performance threshold — films that fall below 85% gloss retention become visibly degraded and the vehicle appearance suffers.

The Ashland PSX Adhesive System

The adhesive system used in Wansen GLS films is the Ashland PSX series of acrylic pressure-sensitive adhesives. Ashland (formerly a division of Lubrizol before the acquisition) has been the dominant supplier of PSA for premium PPF applications for over two decades.

The PSX adhesive is a crosslinked acrylic copolymer that achieves its bond strength through a combination of viscous flow (which creates intimate substrate contact) and covalent crosslinking (which creates long-term durability). Key specifications:

• Peel adhesion (72h cure, stainless steel): >16 N/25mm — this is the figure that determines how well the film resists edge lifting after installation. Below 12 N/25mm, edge lift becomes a significant field problem.
• Shear holding power: The adhesive must not cold-flow (creep) under sustained thermal stress. In GCC summer conditions, a panel can reach 85°C and stay at that temperature for 6–8 hours. The shear holding power of PSX prevents the film from sliding down a vertical panel surface over time.
• Removability: When the film reaches end-of-life, it should be removable without adhesive residue on the paint surface. The PSX formulation allows clean removal with heat assistance and citrus-based adhesive remover, provided the film has not been over-cured (left in place for more than 10 years).

Frequently Asked Questions