# Optimizing the Performance of Window Frames: A Comprehensive Review of Materials in China
## 3 Improvement Methods and Sustainability Assessment
### Polymer  
Polymer materials generally consist of one or more discontinuous phases distributed in a continuous phase, with the continuous phase known as the matrix and the discontinuous phases as additives or reinforcement materials [65; 66; 67; 68; 69]. In polymer materials, components with complementary properties are combined. For example, inserting a reinforcement agent with good heat insulation and high modulus into a polymer matrix makes it possible to enhance mechanical and thermal performance [70; 71]. Polymer materials form systems composed of a relatively large number of components, and these components can be diverse, including organic, mineral, metal, etc. [72; 73; 74]; therefore, the number of polymers that can be produced from this range of basic components is, in practice, limitless. The properties of the matrix, the types of reinforcement and additives, and the formulation ratios of these elements have a decisive impact on the characteristics of the final product, as summarized in Figure 6.  
* Matrix  
The matrix, typically formed in composite materials, serves as the foundation connecting reinforcement fibers, distributing constraints, providing chemical resistance to the structure, and imparting the desired shape to the final product [75; 76]. Thermoplastic matrices have a linear chain structure and undergo a transition in their molten state. In most manufacturing processes, thermoplastic matrices are heated and formed through processes like molding, injection, extrusion, or hot forming. Subsequently, they are cooled, allowing the finished product to maintain its shape; this operation is reversible [77]. Thermoplastic matrices are not easily oxidized, exhibit high corrosion resistance, and serve as excellent thermal and electrical insulators. Due to their light weight, high mechanical strength, and resistance to environmental impacts, thermoplastics are ideal materials for various applications, such as polyvinyl chloride (PVC) [78], polycarbonate (PC) [79], polyetherimide (PEI) [80], and polyamide (PA) [81]. In contrast, thermosetting matrices are harder, exhibit better creep resistance, and are suitable for molding large parts using short fibers, long fibers, or woven fibers [82]. At room temperature, they are usually liquid and undergo a chemical change through heating or the addition of hardening agents, forming highly stable chemical bonds in three-dimensional space; this process is irreversible, and materials treated in this way become insoluble and infusible in most solvents (alcohols, ketones, and hydrocarbons) and demonstrate excellent stability. The most commonly used thermosetting matrices include polyurethane [83], phenolic resin [84], and epoxy resin [85]; polyurethanes are also used for roof windows. An elastic matrix possesses elasticity like rubber. The static elastic body is composed of folded long polymer chains. Under constraints, molecules can slide and deform relative to each other. Typically, to impart good elasticity to the  
Figure 6: Polymer material formulation. matrix, it undergoes vulcanization. During vulcanization, sulfur, carbon, and various chemical agents are introduced into the elastic body [86; 87]. This is a curing process where, without removing the flexibility of the molecular chains, a rigid three-dimensional network is formed. * Reinforcement  
Reinforcement materials, acting as the skeleton of composite materials, provide them with mechanical strength. The most used reinforcement materials are fibers or their derivatives. These materials are made from filamentous substances, including organic fibers or inorganic fibers, ranging from individual particles to continuous fibers [88; 89; 90; 91; 92]. The most used reinforcing fiber is E-glass fiber [93], accounting for over 95% of applications, while carbon fibers and aramid fibers are also widely studied [94; 95]. Generally, in composite structures (anisotropic), fibers exhibit good performance in tension, but their compressive strength and shear resistance are usually weaker [96].