## 1 Introduction  
Dental bleaching is a process of chemical breakdown of chromogens. The recent demand for tooth whitening has increased steeply, making it one of the most sought-after cosmetic dental treatments [1]. The bleaching products are mainly composed of carbamide peroxide (CP) and hydrogen peroxide (HP) compounds as potent oxidizing agents [2]. Despite having a slightly different chemical formulation from HP, CP has a comparable mechanism of action and produces substances that are known to be powerful bleaching agents [3]. The oxidation of chromophores found in enamel and dentin and the breakdown of the extracellular matrix are two of the effects of bleaching. Both of these substances have localized negative effects on the oral mucosa and tooth structures [4].  
Due to quick and effective results, vital teeth whitening procedures are currently among the most sought-after dental procedures [5]. Bleaching procedures can be performedin-office as well as at-home [6]; however, at-home teeth whitening is a common and viable treatment option among patients because of its ease and low cost [7]. The extended and repeated application of over-the-counter bleaching products has been linked to tooth sensitivity, oral mucosal irritation, oral infections, and surface and morphological changes in teeth [8; 9; 10].  
Like clinical studies, laboratory investigations have also demonstrated the adverse effects of bleaching agents. Earlier research has advocated that bleaching agents dramatically diminish the surface gloss of resin composites, affecting the aesthetic quality of restorations [11; 12]. These earlier studies have contemplated the damaging effect of home bleaching gels on the surface roughness [13], microhardness [11; 14], and flexural properties [15; 16] of restorative composites.  
The mechanical properties are considered vital attributes for restorative composites' long-term clinical survivability [17]. Recently, investigators have shifted their interest to employing organic peroxides such as phthaladiporeoxycapric acid (PAP) as the active component in bleaching chemicals, as opposed to alternatives such as HP or CP. PAP is a non-toxic and readily biodegradable agent [18]. An investigation showed that PAP-containing gel caused etched enamel with reduced microhardness [9]. To address these issues, a novel PAP formulation designated PAP+ has been marketed. The formulation of PAP+ includes ingredients such as hydroxyapatite nanopowder and potassium citrate; both of these elicit an effective citrate buffering mechanism that maintains the pH near to neutral during the treatment [18].  
This laboratory study was envisaged to ascertain the consequence of a novel PAP+ bleaching gel on two widely used all-purpose nanohybrid and microhybrid dental composite systems recommended for anterior and posterior restorations [19]. It considered the importance of the surface roughness of a restoration, which could exacerbate periodontal and caries disease [20], ranohardness, which is a valuable method for quantifying the unit area of an indented surface at the nanoscale [21], and high flexural strength and diametral tensile strength for stress-bearing restorations [22]. Taking into account these things, this study aimed to determine the effect of two different home bleaching systems, one based on free radical formulation and the other not, on the organic matrix of the restorative composite material in terms of nanohardness and elastic modulus, as well as to evaluate the surface roughness, flexural strength, and diametral tensile strength of the composite materials. It was hypothesized that the variables (the tested properties of the composite material) would be affected by the bleaching procedure.