Hard coatings for transparent plastics in field testing

Classification: Hard coatings on transparent plastics

Hard coatings increase the abrasion, scratch and chemical resistance of transparent plastics. They protect optical surfaces against haze build-up, micro-scratches and UV-induced ageing, thereby stabilising the optical function over the service life. In industrial applications, the benefit only arises from the combination of substrate, coating system, processing and care. A field test under real ambient conditions closes the gap between laboratory data and practical use.

For polycarbonate, PMMA, polysulfone and polyethersulfone, both the chemical affinity to the hardcoat and the thermal and mechanical boundary conditions differ. The test design must take these differences into account so that the results are transferable.

Relevant substrates and typical fields of application

Polycarbonate as a substrate is impact-resistant, heat-resistant and, without a hard coating, relatively susceptible to micro-scratches. For a robust material classification, the basics on polycarbonate as a substrate help in assessing risks and potentials.

PMMA has a very good optical quality and a higher inherent scratch resistance than PC, but is more sensitive to certain solvents. PSU and PES are high-temperature-resistant, transparent materials for demanding environments with increased thermal and chemical exposure. Technical details on polysulfone (PSU) and polyethersulfone (PES) support the selection for highly stressed visual parts.

Typical fields of application are protective and control windows in mechanical and plant engineering, transparent covers in production lines, housing windows in measurement and testing technology, transport technology and outdoor applications.

Overview of hardcoat systems

The suitability of a hardcoat results from its composition, curing mechanism and layer structure in combination with the substrate and the component geometry.

• Polysiloxane or sol-gel systems (thermally curing): Good abrasion and chemical resistance; often clear, hard layers with a pronounced barrier effect. On the process side, clean pre-treatment and defined oven cycles are decisive.
• UV-curing acrylates: Reactive systems with fast curing, often advantageous for high cycle rates. Formulation and UV dose control hardness, flexibility and adhesion.
• Polysilazanes and hybrid layer systems: Inorganic-organic networks with high density and abrasion resistance. Suitable where, in addition to scratch protection, an increased barrier effect is also required.
• Plasma- or CVD-based thin layers: Very uniform, thin protective layers with high hardness; require substrate- and system-specific process windows.

Practice shows that it is not the nominal layer type but the coordinated combination of pre-treatment, application, curing and post-treatment that determines adhesion, crack resistance and long-term behaviour. Findings from applications with hard-coated polycarbonate sheets in practice can often be transferred to similar substrate/layer combinations.

Field test design for robust statements

A field test supplements laboratory tests with real environmental and cleaning loads. The aim is a statistically meaningful, reproducible test plan with documented exposure and objective evaluation of the relevant parameters.

Specimens, installation situation and exposure

• Number of specimens and variants: Identical geometry and surface quality for each substrate-layer system; sufficient repetitions for scatter assessment.
• Mounting: Same orientation, clamping conditions, distance to contamination sources and air flows as in the target system; representative seals and fastenings.
• Exposure profile: Realistic mixed load of UV, temperature cycling, humidity, dust, splash water, cleaning cycles and mechanical wiping. Events and intervals are logged.

Abrasion and scratch resistance

Laboratory standards such as ISO 9352 or ASTM D1044 (abrasion test with rotating disc) are suitable for evaluation, supplemented by defined wiping tests with standardised cloths. In the field test, representative wiping cycles with real operating media are important. Optical changes (e.g. haze according to ASTM D1003), visible scratch patterns and functional effects such as legibility of displays are recorded. For fundamental comparisons of PC and PMMA, the comparison of various hard coatings on PC and PMMA supports the classification.

UV and weathering resistance

ISO 4892 and ASTM G154 are common references for artificial weathering. They allow comparable ageing under defined spectra and humidity-temperature sequences. In the field test, the real UV dose will not be constant; clear measurement points, photos and accompanying spectral transmission are therefore advisable. A substantive supplement is provided by the overview of transparent engineering plastics for outdoor use.

Cleaning and chemical resistance

Cleaning is a relevant ageing factor. Important groups are alcohol-free cleaners, alcohols (e.g. isopropanol, ethanol), quaternary ammonium compounds, neutral to alkaline surfactant cleaners and occasionally disinfectants or glass cleaners. Tests include wiping tests with defined load, contact time and number of cycles as well as spot tests with covering and subsequent cleaning. Practical recommendations for care are summarised under cleaning and care of transparent plastic glazing.

Optical parameters

For long-term use, total transmission, haze and, if applicable, gloss are relevant parameters. Recording before, during and after exposure allows the distinction between initial surface change and progressive degradation. In addition, colorimetric parameters are helpful when slight yellowing or colour casts are relevant to the assessment.

Adhesion, crack formation and edges

Adhesion tests (e.g. cross-hatch or pull-off tests according to common standards) and micro-crack inspections under oblique light show the interaction of layer hardness, extensibility and thermal expansion. Edges, drilled holes and milled chamfers are critical points. Coating-free edges or burrs can promote creep attack by cleaning chemicals.

Expected differences between PC, PMMA, PSU and PES

• PC: Benefits significantly from hard coatings with regard to scratch events caused by wiping. The toughness of the substrate supports stone-chip and impact events, but requires a layer with sufficient crack resistance.
• PMMA: Good optical starting basis and inherent scratch resistance. With aggressive cleaners, stress cracking can occur, which is why field tests with real media are mandatory.
• PSU: Higher thermal stability and robust mechanics. The substrate colour can appear slightly warm; for colour-critical visual windows, accompanying colorimetric measurements are advisable.
• PES: Very temperature- and media-resistant. The combination of high stiffness and hard layers requires particular attention to crack formation and edge quality.

From these material profiles it follows that the best performance of a hardcoat depends on the substrate-layer compatibility, not just on a single parameter such as pencil hardness. A field-oriented test layout reliably reveals these differences.

Evaluation and decision criteria for practice

Setting up a criteria matrix

A criteria matrix weights abrasion, UV/weathering, cleaning, adhesion, optical stability and process capability. In addition, availability, process windows for component geometry, repairability and supply chain aspects are included.

Defining limit values

• Optics: Upper limits for haze increase and colorimetric deviation, minimum transmission per application.
• Mechanics: Limit image for permissible scratch patterns after defined wiping or abrasion cycles.
• Adhesion: Minimum classification according to the selected adhesion test.
• Cleaning: Approved media list with permissible exposure conditions.

Approval plan and quality control

Pre-series components in the target system, documented cleaning and maintenance procedures as well as periodic inspections ensure transferability. For series monitoring, a lean test plan with samples, optical inspection and indicative tests for adhesion and haze is recommended.

Avoiding typical sources of error in the field test

• Inhomogeneous specimens: Different pre-treatments or edge qualities distort results.
• Non-representative cleaning: Laboratory cleaners instead of real media lead to over-optimistic results.
• Missing intermediate measurements: Endpoint measurements only overlook early degradation phases.
• Inconsistent mounting: Deviating clamps or seals create local stresses.
• Transfer errors: Laboratory results without field comparison are weighted too highly.

Care and cleaning guidelines as a supplement to the hardcoat

An approved list of suitable cleaners, soft cloths, moderate surface pressure and short contact times reduce the risk of scratches and chemical attack. Pressure washers, scouring agents or unsuitable solvents should be avoided. For systems with hygiene or disinfection requirements, a coordinated wiping plan with documented media and interval approval is additionally recommended.

Checklist for the specification of coated visual parts

• Substrate: PC, PMMA, PSU, PES with defined quality, thickness, surface class.
• Hardcoat: System type, application method, curing, target roughness, wet-layer or dry-layer specifications.
• Pre-treatment: Cleaning, activation, adhesion promoters, masking.
• Edges: Machining process, freedom from burrs, any sealing.
• Reference tests: Abrasion and haze tests, adhesion, optical parameters, weathering.
• Cleaning: Approved media, wiping pressure, contact times, cloths.
• Documentation: Specimen setup, exposure, intervals, photographic documentation, measurement records.
• Series monitoring: Sampling plan, complaint and repair guideline.

With such a field test setup tailored to the real application, hardcoat systems on PC, PMMA, PSU and PES can be evaluated comparably, prioritised and reliably transferred into series production.

Frequently asked questions

Which hardcoat types are customary in practice for PC, PMMA, PSU and PES?

Established are thermally curing polysiloxane or sol-gel layers, UV-curing acrylates, polysilazanes and thin plasma/CVD layers. The choice depends on substrate compatibility, process window, abrasion and chemical requirements.

How is abrasion assessed reliably in the field?

Combine standardised laboratory tests (e.g. ISO 9352, ASTM D1044) with defined field wiping tests using real cleaning media. Haze change, visible scratch patterns and optical function (e.g. legibility) are assessed.

Which cleaning agents are generally uncritical?

In many cases, mild, neutral surfactant cleaners and lint-free, soft cloths are suitable. Alcohol- or disinfectant-containing media must be approved in tests depending on the substrate and layer. Scouring agents and hard pads must be avoided.

How long does a hardcoat last outdoors?

The service life depends on UV dose, temperature cycles, cleaning and substrate. Without specific application data, no reliable time statements are possible. A combination of weathering tests and field exposure provides robust statements.

What role do edges and drilled holes play?

Edges, drilled holes and chamfers are frequent initiation points for micro-cracks and undercutting. Clean machining, burr-free edges and, if applicable, edge sealing reduce failure risks. In the field test, such zones should be inspected separately.

Can a damaged hard coating be repaired?

Minor surface scratches can sometimes be polished out, but often lose optical uniformity in the process. Local re-coating must be examined on an application- and system-specific basis. For series approvals, a defined repair guideline is recommended.

How many specimens are advisable for a field test?

The number of specimens should reflect scatter, variants and failure risk. It is customary to use multiple specimens per system per exposure location and time stage. A specific minimum number depends on complexity, safety requirement and acceptance criteria.