How Composite Polymerisation Affects Strength and Colour Stability

Patients watching a white filling being placed see a blue light held against the tooth for ten or twenty seconds. It looks like the simplest part of the procedure.

It is actually the step that determines most of what happens to that restoration over the following years — how strong it is, how well it resists wear, whether it discolours, and how well it stays bonded. Two restorations made from identical material can behave very differently depending on how thoroughly they were polymerised.

This article explains what is happening during that step and why it matters to you as a patient.

Dental composite has three components.

The resin matrix. A mixture of liquid monomers — commonly Bis-GMA, UDMA, TEGDMA and related molecules. These are the part that sets. Our article on Bis-GMA in dental bonding covers the chemistry in more detail.

Filler particles. Glass, quartz, silica or zirconia particles ranging from sub-micron to a few microns. They provide strength, wear resistance and radiopacity, and they reduce shrinkage. Modern composites are typically 60 to 70 per cent filler by volume, and the type and distribution of filler is the main thing distinguishing one material from another.

The photoinitiator system. A light-sensitive molecule, most often camphorquinone paired with an amine, that absorbs blue light at around 460 to 470 nanometres and generates the free radicals that start the reaction.

When the curing light is applied, the photoinitiator absorbs photons and produces free radicals. These attack the carbon-carbon double bonds in the monomer molecules, opening them and allowing the monomers to link into long chains and cross-linked networks.

The liquid becomes a solid. That is the whole process.

The critical measure is the degree of conversion — the percentage of available double bonds that have actually reacted.

It is never 100 per cent. In practical clinical conditions it is typically somewhere between 55 and 75 per cent. As the material stiffens, the remaining monomer molecules become trapped and immobile, and the reaction stops before it is complete.

That residual fraction is not inert, and it has consequences.

Mechanical properties. A higher degree of conversion produces a more densely cross-linked network. Flexural strength, hardness, elastic modulus and wear resistance all improve with conversion. An under-cured restoration is measurably softer and wears faster, which in a load-bearing area translates into a shorter lifespan.

Colour stability. This is the aspect patients notice most. Unreacted monomer and residual unreacted photoinitiator both contribute to discolouration over time. Camphorquinone is itself yellow; it bleaches as it is consumed during the reaction, so incompletely cured material retains more of that yellow tint and the restoration can appear to darken.

Unreacted monomer also makes the matrix more susceptible to water sorption. Water penetrating the resin acts as a vehicle for stains from tea, coffee, red wine and tobacco, and over time contributes to hydrolytic breakdown of the bond between the filler particles and the matrix. Our article on drinking tea and coffee with composite bonding covers the practical implications.

Biocompatibility. Residual monomer can leach from the restoration, particularly in the first day or so. The amounts are small and inadequately cured material is more likely to release more of it, which is another argument for thorough curing.

Wear and surface behaviour. A well-polymerised surface polishes to a higher gloss and retains it longer. A poorly cured surface is softer, roughens sooner, and accumulates more plaque and stain.

Bond strength. The adhesive layer bonding the composite to the tooth is itself a resin that must polymerise. Inadequate curing of the adhesive compromises the seal at the margin, which is the route to marginal staining, sensitivity and recurrent decay.

Light intensity and exposure time. The total energy delivered is intensity multiplied by time. Manufacturers specify a required energy for each material and shade. Halving the intensity requires doubling the time.

Distance from the light tip. Light intensity falls off rapidly with distance. A tip held a few millimetres away from the surface delivers considerably less energy than one held close, which matters most in deep preparations where the tip cannot reach the base.

Increment thickness. Light is absorbed and scattered as it passes through composite, so the deepest part of any increment receives least energy. This is the reason composite is placed in layers, typically no more than two millimetres for conventional materials. Bulk-fill composites use more translucent formulations and modified initiators to allow thicker increments, but even these have specified limits.

Shade and opacity. Darker and more opaque shades absorb more light, so they require longer exposure. Dentine shades and opaquers are more demanding than enamel shades.

Angle of the light. The tip should be perpendicular to the surface being cured. Angled delivery reduces effective energy.

Condition of the curing light. Output falls over time with bulb or LED ageing, and a contaminated or resin-spattered tip can substantially reduce transmission. Lights should be checked periodically with a radiometer. This is one of the less visible aspects of practice quality.

Temperature of the material. Composite stored in a cold place is more viscous, which reduces monomer mobility and slows the reaction. Materials are generally allowed to come to room temperature before use.

Oxygen. Oxygen inhibits free radical polymerisation, which is why the outermost surface layer of any cured composite remains slightly tacky and unreacted. This layer is removed during finishing and polishing — another reason those steps are not merely cosmetic.

As monomers link together, they move closer to one another, and the material contracts. Typical volumetric shrinkage is in the region of 1.5 to 3 per cent depending on the formulation.

That does not sound like much, but the restoration is bonded to the tooth on several surfaces simultaneously. The shrinkage generates stress at the adhesive interface, and if it exceeds the bond strength the result is a gap at the margin, post-operative sensitivity, marginal staining or debonding. In some cases the stress is transferred into the tooth and produces cuspal deflection or cracking.

Techniques used to manage this include incremental layering to reduce the volume shrinking at any one time, attention to cavity configuration, flowable liners with a lower elastic modulus to absorb stress, and soft-start curing protocols that begin at lower intensity to allow some flow before the material stiffens.

Our article on biomimetic layering to prevent composite chipping covers the layering approach in more depth.

The reason this matters to patients is that it explains why restorations placed with the same material can perform so differently, and why time spent on a composite restoration is not wasted.

A carefully layered composite, cured with a light of verified output held close and perpendicular for the specified time on each increment, finished and polished properly, will be stronger, more colour-stable and better sealed than the same material placed quickly in a single bulk increment with a brief light exposure.

There is no way to assess this by looking at a finished restoration on the day it is placed. The difference shows up over years.

• Follow the aftercare advice on staining. The composite continues to mature for a period after placement, and avoiding heavily staining foods and drinks in the first day or two is a reasonable precaution.

• Maintain the surface. Polishing at hygienist visits restores gloss and removes surface stain. Our article on professional polishing for composite bonding explains what this achieves.

• Be careful with whitening. Whitening agents can affect the surface of composite and the bond strength if applied immediately before bonding. Our article on whether whitening weakens composite bond strength covers the sequencing.

• Report marginal staining early. A dark line at the edge of a filling may indicate a marginal gap, and addressing it early can mean a repair rather than a replacement.

• Manage grinding. Heavy forces accelerate wear and chipping regardless of how well the material was cured.

Why does my dentist use the light in several short bursts rather than one long one?

Because composite is placed in increments and each one is cured separately. Light does not penetrate reliably beyond about two millimetres in most conventional composites, so a deep restoration built in one piece would be under-cured at its base — which is where the bond to the tooth is.

Do white fillings really change colour over time?

They can. The causes include residual unreacted monomer, water sorption into the resin matrix, surface roughening that holds extrinsic stain, and dietary factors. Well-polymerised, well-polished composite is considerably more stable than poorly cured material, but no composite is entirely unchanging.

Is a longer exposure always better?

Up to a point. Beyond the energy needed for adequate conversion, additional exposure gives diminishing returns and generates heat, which is a consideration in deep preparations close to the pulp. Manufacturers specify times for a reason, and modern high-intensity lights are not simply faster versions of older ones.

Can a filling be under-cured without me noticing?

At the time, yes — an under-cured restoration looks normal. What may follow over months or years is faster wear, surface roughness, discolouration and marginal breakdown. This is why the unglamorous details of technique matter.

Does the colour of my filling matter for how it sets?

Yes. Darker and more opaque shades absorb more light and need longer exposure to reach the same degree of conversion. This is accounted for in the curing protocol used.

Why does composite need polishing at the end?

Partly for appearance and partly for biology. The outermost layer is oxygen-inhibited and remains unreacted, so it is removed. A smooth, polished surface also accumulates less plaque and resists staining better than a rough one.

Is composite as good as porcelain?

They are different materials suited to different situations. Composite is more conservative of tooth structure, repairable and completed in one visit. Porcelain has better long-term colour stability and surface characteristics and is generally chosen for larger restorations. Our composite bonding and porcelain veneers pages set out the differences.

You are not expected to evaluate curing protocols. What is worth knowing is that the difference between a composite restoration that lasts and one that does not is largely in technique rather than in the material, and that time spent placing it carefully is time well used.

If an existing white filling has discoloured, has developed a dark margin, or feels rough to the tongue, it is worth having assessed — early attention often means a repair rather than a replacement.

You can contact our team to arrange an appointment at our Wimpole Street practice. Our white fillings and composite bonding pages explain what is involved.

This article provides general information about dental materials and does not constitute individual dental advice. The condition of existing restorations and the appropriate treatment can only be determined following clinical examination. All restorations have a finite lifespan and performance varies between individuals depending on bite forces, diet and oral hygiene. Wimpole Dental is regulated by the Care Quality Commission, and our clinicians are registered with the General Dental Council.

Next review due: 12 September 2027