Minimization of the inevitable residual monomer in denture base acrylic

Summary

Objectives

Residual monomer ([MMA]_R) in denture base acrylic continues to be of concern. The response surface of concentration vs. time and temperature for the equilibration of methyl methacrylate (MMA) and its polymer (PMMA) allows a prediction of the time to the minimum at any temperature for a closed system. It was the purpose here to determine whether this prediction applies to normal denture base processing, and whether optimum conditions could be identified.

Methods

Denture bases were processed following normal laboratory procedures, including pre-cure for 3 h at 70 °C for all tests. Commercial powder and liquid were used at either 95 or 100 °C, or a plain PMMA powder and the same liquid at 95 °C, for times ranging from 5 to 192 h. Residual MMA was determined by gas chromatography.

Results

[MMA]_R decreased steadily from ∼0.25% to as low as ∼0.07% with increasing time at temperature, but did not approach equilibrium. The rate of diffusive loss of MMA appears to exceed the rate of depolymerization.

Significance

Residual monomer is inevitable for all PMMA-based products no matter what the curing conditions are. However, extended time at high temperature can allow low values to be attained, and the time allowed can compensate for processing temperatures somewhat lower than the ordinarily recommended 100 °C. It is suggested that overnight processing at 95 °C should be adopted to minimize [MMA]_R and save energy. This result is of importance for work at high altitude.

Introduction

It has long been known that there is residual monomer left in denture base acrylic after processing, with many reports on the biological and mechanical consequences since the use of poly(methyl methacrylate) (PMMA) became widespread in about the 1950s. This monomer, methyl methacrylate (MMA), diffuses out of the denture and thus into adjacent oral tissues; this may result in irritant and allergic reactions, including a ‘burning mouth’ sensation (BMS). Numerous studies have dealt with such leaching [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12] and the biological effects of MMA on the acrylic denture-wearing patient [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. Broadly, it was found that the amount leached was proportional to the amount of residual MMA (MMA_R) present, so more was leached from cold-cured resins. Most MMA_R was lost in the first few days of immersion but became almost constant after 2 weeks. This diffusive loss of MMA_R was also found to increase with immersion temperature.

Skin-patch testing for ingredients of acrylic denture materials such as PMMA, MMA, benzoyl peroxide and hydroquinone has shown mild to strong reactions toward MMA, which was therefore concluded to be an allergen [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. In addition, the mechanical properties of the polymer are affected by MMA_R concentration, [MMA]_R: tensile strength, modulus of elasticity and surface hardness have been found to be lower with greater [MMA]_R [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34]. The explanation is the plasticizing effect of the MMA. Therefore, it is desirable on both accounts to reduce the value of [MMA]_R in the processed denture to the point where both biological and mechanical effects are negligible.

There have been many studies of the effect of curing conditions on the value of [MMA]_R in denture acrylic (Table 1). Thus, the effects of temperature, time, initiator concentration, curing environment, water bath vs. oven, pressure and mixing ratio (polymer: monomer) have been investigated. In summary, it has been reported that higher curing temperature and longer curing time lower the value of [MMA]_R [33], [35], [36], [37], [38], [39], [40], [41]. Curing under pressure was said to allow shorter processing time with [MMA]_R comparable with that for the more usual long cure without the formation of porosity [42]. It has been claimed that increasing the proportion of monomer in the mixture decreased [MMA]_R on the basis that there was greater temperature rise during polymerization such that more monomer reacted [27]. This ignores the increased polymerization shrinkage that would result. Of the many curing cycles studied, 70 °C for 7 h+‘terminal boil’ was concluded to give the lowest value of [MMA]_R [27], [38], [39], [40].

However, what is curious about this large literature is that there is nowhere a mention of the essential equilibrium between monomer and polymer in such a free-radical polymerization system. It seems to have been implicit that (a) MMM_R is merely a consequence of inadequate processing that could be fixed if only the correct conditions could be identified, and (b) that a target of zero for [MMA]_R was realistic, although never known to be attained. Furthermore, there is nowhere in the dental literature on this topic or in text books, as far as is known, any mention of the ordinary chemical considerations of the effects of temperature on the rate of reaction, or indeed of the expected progress of the reaction with time. Given that free-radical polymerization has been well-enough understood for a long time, this is all the more extraordinary.

A systematic investigation of [MMA]_R against time and temperature was recently reported [43] in which the following equation was deduced as broadly representing the response surface for the reaction in a sealed system:

$$\text{log}[M]=\text{log}\left[\frac{[M_0]}{2}\text{erfc}\left[\frac{z+b}{c}\right]+d\times {10}^{ze}\right]$$

where

$$z=a\text{log}(t)-\frac{1000}{T}$$

Here, [M₀] is the initial concentration of MMA, t is the time in h and T the temperature in K; logarithms are to base 10. The values of the fitted constants and their standard errors are given in Table 2. The locus of the fitted surface minimum with respect to either variable is shown in Fig. 1. Two points may be made immediately: the time to attain the minimum at 70 °C is in excess of 300 h; and that at 100 °C around 24 h would be required. The contrast with normal processing conditions is marked.

The questions then arise as to whether (a) this result (i.e. Eq. (1)) was applicable to the effectively open system of normal denture base processing (i.e. the mould is porous), (b) whether the minimum is accessible in a reasonable time with available equipment and other normal operating circumstances, (c) what the minimum might be in such a context. Accordingly, it was the purpose of this work to study the effect of processing time and temperature on the value of [MMA]_R in PMMA denture bases in the light of the predictions made from Eq. (1).