**4.1. Desirable characteristics of poly(methyl methacrylate)**

Dental prostheses are defined as an artificial supplement used to complement or to restore missing teeth. These can be made from a variety of materials including metals, ceramics or plastics. Initially, market of dental products was predominated by ceramic restorations. However, with the development of materials science and polymer technology, ceramics has been supplanted by plastics in this field of medicine [16].

The first dentures based on poly(methyl methacrylate) appeared in the 1930s of the twentieth century. Nowadays, the vast majority of dentures is based on poly(methyl methacrylate) or on the mixture of this polymer with copolymers of methyl methacrylate, butyl methacrylate, or propyl acrylate [17–19].

In dental prosthetics, resins based on acrylates are widely used. What is important, these can be used in the form of a powder or liquid. Such compositions also consist of dibutyl phthalate, acetone as well as a variety of stabilizers [19].

Poly(methyl methacrylate) is the most commonly used polymer. It is a result of many desirable characteristics of this compound with a particular emphasis on high mechanical strength, crack and abrasion resistance, acid resistance as well as the ability of easy coverage of this polymer with selected pigment. Furthermore, economic considerations, ease of application, aesthetics as well as optical properties or biocompatibility are undoubtedly important features speaking in favour of the application of this polymer [13, 17].

Resins based on the above-mentioned polymer show a number of advantages whereby they constitute the desired raw material for the preparation of dentures. Therefore, such materials are used to obtain full and removable partial dentures, prostheses for maintaining and microdentures [20].

## **4.2. Modifications of poly(methyl methacrylate)**

Poly(methyl methacrylate) is characterized by some imperfections that should be modified or improved. One of the mentioned imperfections of these polymers are undoubtedly mechanical properties. Over 60% of dental prostheses is broken or cracked within the first 3 years of use. The repair process of damaged dental materials is time consuming and costly. It should be noted, however, that the vast majority of users of such materials are elderly people for whom subsequent visits in the dentist office constitute a troublesome duty. Hence, the improvement of the mechanical properties of prostheses running with simultaneous maintaining of other desirable from the point of view of the patient characteristics becomes a real challenge for such realms as materials science and dentistry [13, 21].

An important disadvantage of poly(methyl methacrylate) is a tendency of microorganisms to adhere to its surface which contributes to the microbial growth on the prosthesis made of this material. One of the results of such phenomena is the occurrence of inflammation in the oral cavity. It is also worth noting that dentures based on poly(methyl methacrylate) are characterized by a significantly larger resistance to wear in comparison to ceramic restorations. Hence, many attempts are undertaken in order to improve the resistance which can be achieved, for example, by increasing the cross-linking degree of the polymer or by change of conditions of the polymerization process.

It is possible to distinguish three methods that can solve the problem of PMMA imperfections. These are the


Improvement of the physico-chemical properties of acrylates can also be obtained by copolymerization as well as by development of blends or interpenetrating polymer networks based on poly(methyl methacrylate). An impact on the characteristics of the mentioned acrylic resins has undoubtedly addition of various types of organic or inorganic compounds that takes place during process of the polymerization [22].

On the basis of the research, it was found that the introduction to the resin based on bisphenol A glycidyl methacrylate (BIS-GMA) inorganic fillers of micrometric size causes a significant improvement in the strength properties of the material. Moreover, the same effect is reached by addition of inorganic fillers having a nanosize to the matrix based on poly(methyl methacrylate) [24, 25].

An interesting modification of resins based on PMMA is the addition of titanium dioxide. The nanoparticles of this inorganic oxide contribute to the antibacterial properties of the resin. These properties are the result of cytotoxic effects of oxide radicals generated by treating the titanium compound with UV radiation. Furthermore, it was found that the antibacterial activity of TiO2 is additionally intensified by the presence of metal or metal oxide (e.g. Fe), and therefore, Acosta-Torres et al. carried out a synthesis of the resin based on poly(methyl methacrylate) containing nanoparticles of TiO2 and Fe2 O3 . Anehosur et al. came to the same conclusion. In the research, they have identified the biocidal properties of the acryl polymer resin modified with titanium dioxide in relation to the selected bacterium, i.e. *Staphylococcus aureus*. Numerous studies and observations conducted by these scientists lead to the conclusion that titanium dioxide subjected to an appropriate radiation effects on the inhibition of the growth and the development of the mentioned strain of bacteria [17, 26, 27].

Next interesting additive that has an impact on mechanical properties of acryl polymer resins is zirconium oxide. Gad et al. on the basis of their research concluded that introduction of the mentioned oxide improves significantly flexural strength of described dentures. Better effect is observed by the introduction into acrylic resin zirconium oxide in the form of nanotubes as evidenced by the conclusions drawn on the basis of the research of Yu et al. [19, 28].

It has also been proved that the addition of substances such as nitrile rubber or materials of ceramic origin such as aluminium oxide affects the improvement of properties such as impact strength, fracture strength or hardness. Such conclusions have been reached by Alhareb et al. [29].

An interesting addition to the acrylic resins forming of the base of dental prostheses represents nanoparticles of gold, silver and platinum. These substances give the mentioned material antifungal properties. This addition is intended to prevent the occurrence of fungal diseases as well as to improve oral hygiene. Promising effect was observed in the case of the introduction of nanoparticles in an amount of 2.0 wt.%. Then, the most visible antiadherent effect was observed in relation to the fungi of the genus *Candida albicans* [30].

cavity. It is also worth noting that dentures based on poly(methyl methacrylate) are characterized by a significantly larger resistance to wear in comparison to ceramic restorations. Hence, many attempts are undertaken in order to improve the resistance which can be achieved, for example, by increasing the cross-linking degree of the polymer or by change of conditions of

It is possible to distinguish three methods that can solve the problem of PMMA imperfections.

Improvement of the physico-chemical properties of acrylates can also be obtained by copolymerization as well as by development of blends or interpenetrating polymer networks based on poly(methyl methacrylate). An impact on the characteristics of the mentioned acrylic resins has undoubtedly addition of various types of organic or inorganic compounds that takes

On the basis of the research, it was found that the introduction to the resin based on bisphenol A glycidyl methacrylate (BIS-GMA) inorganic fillers of micrometric size causes a significant improvement in the strength properties of the material. Moreover, the same effect is reached by addition of inorganic fillers having a nanosize to the matrix based on poly(methyl methacrylate) [24, 25]. An interesting modification of resins based on PMMA is the addition of titanium dioxide. The nanoparticles of this inorganic oxide contribute to the antibacterial properties of the resin. These properties are the result of cytotoxic effects of oxide radicals generated by treating the titanium compound with UV radiation. Furthermore, it was found that the antibacterial

and therefore, Acosta-Torres et al. carried out a synthesis of the resin based on poly(methyl

conclusion. In the research, they have identified the biocidal properties of the acryl polymer resin modified with titanium dioxide in relation to the selected bacterium, i.e. *Staphylococcus aureus*. Numerous studies and observations conducted by these scientists lead to the conclusion that titanium dioxide subjected to an appropriate radiation effects on the inhibition of the

Next interesting additive that has an impact on mechanical properties of acryl polymer resins is zirconium oxide. Gad et al. on the basis of their research concluded that introduction of the mentioned oxide improves significantly flexural strength of described dentures. Better effect is observed by the introduction into acrylic resin zirconium oxide in the form of nanotubes as

It has also been proved that the addition of substances such as nitrile rubber or materials of ceramic origin such as aluminium oxide affects the improvement of properties such as impact strength, fracture strength or hardness. Such conclusions have been reached by Alhareb et al. [29].

evidenced by the conclusions drawn on the basis of the research of Yu et al. [19, 28].

growth and the development of the mentioned strain of bacteria [17, 26, 27].

is additionally intensified by the presence of metal or metal oxide (e.g. Fe),

O3

. Anehosur et al. came to the same

and Fe2

the polymerization process.

34 Acrylic Polymers in Healthcare

• Replacement of PMMA by other polymers

place during process of the polymerization [22].

methacrylate) containing nanoparticles of TiO2

• Chemical modification of PMMA by grafting with other polymers

• Introduction to the matrix based on PMMA additives of differ origin [22, 23]

These are the

activity of TiO2

Pan et al. on the basis of their research have been stated that the introduction of hydroxyapatite into PMMA-based resins also results in an improvement of mechanical properties of acrylic dentures made of such composite. Hydroxyapatite is an inorganic compound characterized by the properties that are desired in medical applications, i.e. non-toxicity or biocompatibility. Recently, this interesting material constitutes an addition to the polymer matrix that contributes to the improvement of its mechanical properties [31].

Acrylic materials used in dentistry are commonly reinforced by means of different types of fibres wherein special attention should be paid on ceramic, carbon and glass fibres. Their introduction into the material is aimed towards the improvement of the material resistance to severe stress. Kostoulas et al. [32] in the framework of the research described the effect of the addition of glass fibres on the properties of acrylic denture. Similar conclusions have been drawn by Narva [33] based on research undertaken in the framework of the doctoral dissertation concerning strengthening of dental prostheses.

Analysis of the results allows the conclusion that the presence of glass fibres greatly affects the mechanical properties of the tested prostheses. Significantly greater impact resistance of reinforced dental restorations was observed. Furthermore, dentures modified with glass fibers demonstrated more resistance to cracking. What is essential, it was found that the modification using the described fibers has a better impact on the prosthesis than the addition of metal elements [12, 31–33].

Besides glass fibers, the preferred addition to the dental restorations also provides aramid fibres. Raszewski [12] proved that the modification of the denture with this kind of fibres contributes to a clear improvement in mechanical strength of the tested material. Braden et al. [34] in their publication also drew attention to the effect of the addition of the polyethylene fibres to the prosthesis on its properties. These fibers are characterized by the superior properties in comparison to the previously described glass fiber. However, due to the difficulty of combining them with the acrylic resin, it is necessary to use a low-temperature plasma in order to enable synthesis of this composite [12, 34].

Balos et al. [35] in their research undertook the characteristics of acrylic resins modified with nanosilica. An analysis of the obtained materials included defining of mechanical properties such as elasticity or bending strength. The researchers also determined the cytotoxicity of modified materials. Based on the analysis, it was found that for most of the tested prostheses, addition of SiO2 nanoparticles resulted in an increase of bending strength and in improvement of the modulus of elasticity. It is noteworthy that the amount of added nanosilica is very important because introduction of too much amount causes that material becomes toxic [35].

## **4.3. Preparation of dentures on the basis of poly(methyl methacrylate)**

Poly (methyl methacrylate) constitutes a raw material for the preparation of artificial teeth. However, this polymer to be able to be used for this purpose should be characterized by a high molecular weight and a high cross-linking degree. The starting material consists of PMMA or a copolymer of the mentioned polymer with 1,4-butanediol dimethacrylate or with ethylene glycol dimethacrylate [12].

Crucial meaning for the quality of the denture has a method of polymerization. It is generally carried out at elevated temperature. However, for some substrates that have a tendency towards spontaneous polymerization, process must be carried out at room temperature [12, 36].

Preparation of artificial teeth from the raw material which is PMMA is a multistep process. Initially, 90% of the mixture is represented by monomers, and the remaining 10% constitutes a pigment selected depending on the desired colour of the final product. Depending on the requirements of the final material, it is possible to obtain colourless acrylic mass or mass with the white or pink colours. White acrylic mass is then used to produce dental crowns, inlays or artificial teeth, while the pink one is useful in obtaining products such as impression trays [12, 36].

The next step is subjecting the mixture of trituration process by means of ball mills. Such process depending on the properties of applied reagents takes 24–48 h. Homogeneous mass after trituration process is left in a closed container until reaching the material of gum consistency. Such a mass is then divided into smaller parts which are distributed to the appropriate forms. The moulds are then assembled and subjected to pressing at elevated temperature. At high temperature and under elevated pressure, polymerization process takes place, and final product is obtained. Next, obtained polymer is gradually cooled and formed into its final form by cutting off unpolymerized portions. Subsequently, obtained teeth based on PMMA are affixed to the polymeric plate. An alternative method for the preparation of artificial teeth is a method using the process of injection into the mould at elevated pressure. Molten polymer mixed previously with pigment is subjected to this process [12, 36].

Prepared dental prostheses are characterized by a lack of smell, and furthermore, their sizes can be easily adjusted according to the user [36].
