Comparison of hard coatings on PC and PMMA for industrial visual components

Initial situation and objective of this comparison

Transparent viewing parts in machines, control panels and protective cladding must remain optically clear, be cleaned frequently and withstand UV exposure in outdoor areas. Hard coatings on polycarbonate (PC) and polymethyl methacrylate (PMMA) address abrasion, chemical attack and weathering for this purpose. This article compares key performance characteristics and processing practice so that you can make an informed decision about which combination of base material and coating is suitable for your application.

Clarification of terms: hard coating refers to transparent, abrasion-inhibiting and usually chemical- and UV-stabilising top layers in the micrometre range, which are applied to sheets, moulded parts or films by dip, flow, spray or plasma-assisted processes.

Hard coating types and their relevance for PC and PMMA

Typical systems

• Polysiloxane or organically modified silicate coatings with high scratch resistance and good cleaning resistance.
• UV-curing acrylate coatings with fast curing and balanced abrasion and chemical performance.
• Sol-gel systems with an inorganic content for increased surface hardness and weathering stability.
• Plasma or PECVD layers as thin inorganic barriers with a very homogeneous surface.

Adhesion and pre-treatment

PC is softer and more susceptible to stress cracking than PMMA. For durable adhesion, matched primers and careful surface preparation are decisive. PMMA often shows good inherent adhesion of systems compatible on the coating side. In both cases, cleanliness, defined drying and controlled curing are key quality factors.

Abrasion and scratch resistance in comparison

Baseline level of the base materials

• PC: high impact strength, but a comparatively soft surface. Without a coating, micro-fine scratch structures quickly form under cleaning and particle contact.
• PMMA: inherently more scratch-resistant than PC, but also susceptible to an increase in haze under continuous contact with abrasive media.

Effect of the hard coating

On PC, the gain in scratch and abrasion resistance is typically particularly pronounced, as the coating effectively shields the soft substrate surface. On PMMA, the coating improves the already higher baseline level and stabilises the optics under cyclic cleaning. For objective assessment, Taber abrasion tests are used as is standard in the industry, for example in accordance with ASTM D1044 or ISO 9352. They compare increases in haze under defined conditions. Specific target values should be determined on a project-specific basis with the coating supplier.

Chemical resistance in comparison

Relevant media

In practice, cleaning and disinfecting agents, alcohols, mildly alkaline or mildly acidic media and occasionally fuels or cooling lubricants dominate. Both PC and PMMA are sensitive to certain solvents. Hard coatings act as a barrier and facilitate repeated wet cleaning.

PC plus hard coating

PC benefits greatly from the barrier effect provided by the coating. Stress cracking caused by critical cleaning agents can be reduced, provided the coating is intact and edges have been processed properly.

PMMA plus hard coating

With PMMA, the focus is on increasing resistance to cleaning and wiping. Against strongly solvent-containing media, as with PC, product-specific compatibility tests are advisable. Tests can be based on ISO 175 or internal approval protocols.

A realistic list of media is important. General statements do not replace verification under the concentrations, temperatures and cycle numbers actually used.

UV and weathering stability

Material properties

• PMMA is generally well suited to outdoor applications; it exhibits stable transparency under UV exposure.
• PC requires UV-stabilised grades for outdoor use and additionally benefits from hard coatings that incorporate UV absorbers or provide an inorganic top layer.

Role of the coating

Hard coatings can significantly improve yellowing and haze stability outdoors. For comparability, weathering tests in accordance with ISO 4892 are used. The combination of UV-stabilised PC and a UV-effective hard coating is appropriate for exposed viewing parts. With PMMA, the coating consolidates cleaning and abrasion stability over long service lives.

Processing and manufacturing practice

Cutting, edge processing and drilling

• Coated surfaces are more brittle than the substrate. Tools with a fine cutting edge geometry, reduced feed rates and clean support minimise chipping at the coating edge.
• Production without rework is ideal. Where unavoidable, protected surfaces and film covering until assembly are advantageous.

Thermoforming and bending

The shaping of coated sheets is limited depending on the system. Minimum bend radii and permissible degrees of forming are manufacturer-specific. Many systems require the forming of uncoated semi-finished products with subsequent coating of the moulded part. Alternatively, bendable coatings are used which permit defined radii. Matched drying and curing reduces the risk of microcracks.

Printing, marking and bonding

• The adhesion of printing inks or adhesives on hard coatings is significantly different from that on bare surfaces. Adhesion primers, suitable inks and a preliminary test are obligatory.
• Bonding via uncoated masking areas or backing constructions with integrated seals are proven alternatives.

Laser and waterjet processing

Laser processes can produce local discolouration or edge burrs on coated surfaces. Waterjet cutting avoids thermal influences and is often more process-reliable for finished top layers. Cutting parameters should take sample parts into account.

Optical properties and function

Transmission, haze and anti-reflection

High-quality hard coatings change transmission only slightly. They reduce the increase in haze caused by micro-scratches over the service life. Options such as anti-glare, anti-reflective or anti-fog are available as coating variants and should be clearly named in the specification.

Cleaning cycles and resistance

For applications with frequent wet cleaning, a combination of a hard top layer and suitable chemical resistance is decisive. Edge design and drilled holes are potential weak points. Clean edge processing all the way round protects against undercutting.

Testing, standards and specification

• Abrasion resistance: increase in haze after Taber in accordance with ASTM D1044 or ISO 9352, stating the type of wheel and number of cycles.
• Weathering: ageing in accordance with ISO 4892, stating the method and exposure duration.
• Chemicals: media test in accordance with ISO 175 or application-specific factory standards, defined concentration, temperature and cycles.
• Adhesion: cross-cut or scribe test by the agreed method, before and after ageing.
• Optics: initial transmission and haze, document the measurement geometry.

Comparison overview by application criterion

Criterion	PC with hard coating	PMMA with hard coating
Abrasion and scratch resistance	Very high gain compared with untreated, good for frequent cleaning	Significant improvement, stabilises an already good baseline level
Chemical resistance	Barrier effect reduces stress cracking, check media list	Increases wiping and cleaning resistance, check media list
UV and weathering stability	Suitable for outdoor applications with a UV-effective layer	Very good for outdoor applications, coating increases long-term stability
Impact strength	Very high, even after coating	Limited compared with PC, check for impact loads
Processing	Shaping often before coating, plan rework carefully	Similar, sometimes greater scope, observe system dependency
Optical options	Glossy, anti-glare, anti-fog available depending on system	Glossy, anti-glare, anti-fog available depending on system

Practical selection guidance

• High impact loads plus frequent cleaning: choose PC with a robust hard coating and UV stabilisation.
• Maximum UV transparency and high-quality optics: PMMA with hard coating, if there are no extreme impact loads.
• Contact with cleaning solvents: verify the media list with the coating supplier and ensure edge quality.
• Complex 3D moulded parts: design the process chain for preforming without coating with subsequent coating, or specify a bendable coating.
• Control fronts with touch: combine surface hardness, anti-glare and finger abrasion resistance in a matched coating.

Checklist for specifications

• Define base material, thickness, optical quality and protective film.
• Name the coating side(s), layer system and desired additional functions.
• List abrasion, UV and chemical tests with standard, parameter and target criteria.
• Specify processing steps, minimum bend radii and masking areas.
• Document the cleaning instruction and approved media.

Further resources

• Practice with hard-coated polycarbonate sheets
• Transparent plastics in outdoor areas
• Laser processing of transparent plastics
• Waterjet cutting vs. laser processing
• Overview of chemical resistance
• Polycarbonate material page
• Transparent engineering plastics
• Overview of processing methods

Frequently asked questions

Which minimum bend radii must be observed with hard-coated PC or PMMA?

Minimum bend radii are coating-specific. Many systems require larger radii than the untreated substrate. Clarify radii, forming temperature and curing with the coating supplier using sample parts.

Is a hard coating on both sides advisable, or is one side sufficient?

For control fronts, a one-sided coating on the exposed side is often sufficient. For outdoor glazing or parts cleaned from both sides, a double-sided coating is appropriate.

Can a damaged hard coating be repaired locally?

Minor scratches can usually not be recoated locally. Replacement or complete recoating of the component is the reliable way to restore optical homogeneity and protective performance.

Which cleaning agents are suitable for hard-coated surfaces?

Mild, solvent-free cleaners and soft cloths are the first choice. Check alcohol- or surfactant-containing media against the coating supplier's approval list and avoid abrasive aids.

How are coating requirements specified in a verifiable way?

Define the standard, parameter and target criterion. Examples are increase in haze in accordance with ISO 9352, weathering in accordance with ISO 4892 with exposure time, chemical testing in accordance with ISO 175 and adhesion before and after ageing.

Is laser processing suitable for already coated sheets?

Laser can produce local discolouration or edge burrs. For finished coatings, waterjet cutting is often more process-reliable. Samples and matched process parameters are decisive.

What role does edge quality play for chemical resistance?

Cleanly processed, polished or fine-milled edges reduce the risk of undercutting and stress cracking. Open or chipped coating edges must be avoided.

When is PMMA less suitable than PC despite coating?

Under high impact and shock loads, for example on machine guard windows, PC with hard coating is usually the more robust choice. PMMA scores on optics and UV, but does not achieve the impact strength of PC.