Optimizing the Performance of Window Frames: A Comprehensive Review of Materials in China

Abstract

As the construction industry places increasing emphasis on environmental conservation and sustainability, this trend has spurred profound research into the optimization of door and window performance. One of the critical components of windows is their frames. Over the past several decades, the design of window frames has undergone significant innovations, ranging from introducing new materials to novel design concepts. The performance of window frames is typically influenced by materials, structural design, and the surrounding environment. Consequently, this paper analyzes the common window frame materials in Chinese civil buildings through investigation. It explores commonly used types of window frames available in the market, focusing on their materials and structural designs. It analyzes issues observed during their usage, integrates findings from existing research, and discusses the performance of window frame materials. Additionally, it explores improvement strategies to meet the evolving demands of contemporary and future architectural doors and windows, providing valuable reference points for designers. Finally, approaching the discussion from a sustainable development perspective, the paper evaluates the environmental impact of wood, aluminum alloy, polymer, and composite window frame materials. It emphasizes that wood- and aluminum-clad wood windows represent sustainable options with versatile applications in diverse scenarios.

1. Introduction

The escalating global environmental crisis is increasingly pronounced, with the construction industry consuming 50% of the world’s electricity and contributing to 38% of global carbon emissions [1,2]. Figure 1 illustrates the environmental impact of windows, one of the building elements, and their frame materials throughout their life cycle. Typically, windows only occupy 1/8 to 1/6 of the building envelope area, but their heat dissipation can reach approximately 40–50% of the total building envelope [3]. Simultaneously, the energy content of the materials involved and their environmental impact accentuate the significance of windows in the ecological context of construction. The improvement of energy-efficient and eco-friendly windows can directly reduce environmental impacts. Therefore, minimizing the ecological burden throughout their entire life cycle is crucial for achieving a general level of sustainable development, particularly within the realm of buildings.

One of the most crucial elements of a window is its frame, which not only ensures the structural integrity of the window system but also plays a significant role in energy efficiency and environmental conservation. Over the past several decades, the design of window frames has undergone significant innovations, ranging from the introduction of new materials to novel design concepts. Wooden frames are commonly used in residential construction. It is well-known for its high strength and relatively good thermal performance; however, wooden frames are prone to cracking, warping, and moisture damage, leading modern frame designs to explore alternative materials.

Due to its low material cost, light weight, and high strength, aluminum frames have become prevalent. Aluminum window frames dominate the market in high-rise buildings, as these structures require more robust frames. However, aluminum has high thermal conductivity, making it the least thermally efficient among commonly used frame materials. Another option is polymer frames, typically made from polyvinyl chloride (PVC), unplasticized polyvinyl chloride (UPVC), or glass fiber-reinforced polyurethane (GFRP). However, they have limitations when it comes to size and weight since they are not inherently sturdy. GFRP, with a strength eight times that of PVC, offers advantages in terms of moisture resistance, corrosion resistance, and resistance to UV degradation. Nevertheless, GFRP thermal conductivity is nearly twice that of wood and PVC [4]. From this, it is evident that there is no perfect window frame material. High-performance frames often explore composite material options, such as vinyl-clad aluminum or aluminum-clad wood.

The work of this review addresses issues arising from the use of modern window frame materials. It analyzes improvement methods employed by researchers from the past to the present and conducts a sustainable re-evaluation of materials. The aim is to provide a reference for architectural designers. Following a brief introduction in Section 1, Section 2 delves into the material performance of wooden window frames, polymer window frames, and aluminum alloy window frames, respectively. Section 3 addresses the issues encountered during the use of window frames; this review surveys improvement methods and provides a summary. The environmental impact of different window frame materials is also discussed in the context of their entire life cycle, accompanied by a sustainability assessment. Finally, Section 4 serves as a conclusion to the overall survey, offering recommendations for future work.

2. Material Characteristics

2.1. Wooden Window Frame

Wooden window frames, traditionally crafted from wood as a raw material, exhibit high strength and excellent thermal insulation performance [5,6]. In civil buildings, coniferous wood, such as camphor pine and larch, is commonly used as the preferred material for window frames. However, when multiple factors such as water, heat, light, microorganisms, and loads act in concert, issues arise, including low dimensional stability, susceptibility to decay, and flammability. These shortcomings stem from the inherent structure of wood, comprising macroscopic organizational structures, microscopic cell wall structures, and ultra-microscopic wall layer structures [7]. The deterioration of these structures and components can significantly impact the normal functionality of doors and windows. Wooden materials exhibit a hierarchical structure, extending from the macro-scale to the nanoscale, as illustrated in Figure 2. The primary chemical components of wood form the substances constituting the cell wall and the inter-cellular layers. These components, mainly cellulose, hemicellulose, and lignin, represent three high-molecular-weight compounds that collectively create a natural fiber-reinforced composite material. The properties of this material depend on the arrangement of cellulose fibers embedded in the lignin and hemicellulose matrix. At the macro-scale, wood consists of different types of cells with varying volume fractions, including vessels, fibers, and micrometer-sized pits that traverse cell walls [6,8,9]. At the cellular and tissue levels, the layered porous structure of wood is formed by nanosized pores between cellulose micro-fibrils in the cell wall, creating a complex and hierarchical organization [10,11,12].

2.2. Polymer Window Frame

Another option for window frames is the use of polymers, including polyvinyl chloride (PVC), unplasticized polyvinyl chloride (UPVC), or glass fiber-reinforced polyurethane (GFRP). Window frame polymer materials have good insulation and thermal insulation properties. The insulation performance of window frames mainly comes from the utilization of the insulation properties of polymers themselves. However, polymer materials are susceptible to various factors such as loads, heat, oxygen, water, light, and chemical agents; under the influence of these factors, the chemical composition and structure of the materials change, a behavior referred to as aging [13,14,15]. Aging is related to external environmental factors and is also associated with internal chemical composition, internal structure, and molecular weight distribution; the macroscopic effects of aging can be reflected in changes in certain properties of the material. For example, when it comes to dimensions and weight, polymers have limitations because they are not inherently robust. Figure 3 illustrates the primary mechanisms of polymer aging under mechanical stress (taking tensile stress as an example); when stress is applied to polymer materials, the microstructure of polymer chains changes, causing the system to dissipate mechanical energy through relaxation until the externally applied stress exceeds a certain limit, at which point relaxation can no longer fully offset the mechanical energy, leading to material fracture [16]. To prevent creep of window frames under long-term constant load conditions, for polymer materials and composite materials, some inhibitors can be added to slow down the occurrence of creep.

2.3. Metal Window Frame

Compared to the natural flaws of wood, polymers exhibit brittleness. In modern construction, attention has often shifted to alternative materials, and aluminum alloys, with their durability, lightweight nature, and high strength, are widely used in window frames. However, aluminum alloys are susceptible to sudden deformation and damage during use due to various mechanical and environmental factors, including strong winds, seismic loads, impact forces, and other mechanical loads [17,18]. Typically, when aluminum alloys are exposed to the air, a layer of aluminum oxide forms on the metal surface, tightly adhering to the aluminum alloy surface. This oxide layer helps prevent corrosion; however, under certain conditions, such as exposure to specific chemicals or high humidity, the protective oxide layer may break down, leading to the corrosion of aluminum alloys [19,20,21,22]. In the corrosion process of aluminum windows, moisture is one of the most significant influencing factors [23]. Figure 4 illustrates the typical process of aluminum corrosion, starting with the transfer of charges and concluding with the formation of aluminum oxide in this electrochemical corrosion process. Firstly, aluminum is oxidized to aluminum ions in the anodic region, and the electrons released from the oxidation reaction migrate to the cathodic region. In this region, oxygen acts as the oxidizing agent that facilitates the cathodic reaction, and the corrosion rate is strongly influenced by the oxygen content in water, and the oxidation reaction also depends on the pH of the solution. Taking the reaction in neutral or alkaline solutions as an example, oxygen in the cathodic region gains electrons, followed by a reduction reaction. Apart from humidity, aluminum windows are highly susceptible to corrosion in chloride-containing media; the introduction of chloride ions directly promotes the corrosion process of aluminum by altering the corrosion potential, especially in coastal and marine areas [24,25,26]. These corrosive factors participate in the electrochemical corrosion of aluminum, including pitting [27], crevice corrosion [28], and galvanic corrosion [29]. Additionally, metals exhibit high thermal conductivity, and among commonly used framing materials, aluminum has the poorest thermal performance [30].

3. Improvement Methods and Sustainability Assessment

3.1. Wood

• Adhesive technologies

The moisture content of wood significantly influences its properties, and within the moisture range from bone-dry to the fiber saturation point, the uptake or release of moisture induces dimensional changes in the wood cell wall [31,32,33]. In current applications where wood is used as a material for window frames, it is often processed into glulam, which improves the dimensional stability and durability of the wood. Glulam is typically based on solid wood, and the production involves cutting, grading, drying, and sterilization as pre-treatments; high-performance adhesives are then used to bond the wood pieces in a specific arrangement, as depicted in the typical process flow in Figure 5. For glulam, it is very common to have window frames consisting of three or four lamellas in thickness. The production process of glulam determines that its material properties are closely related to the properties of bonded layer wood and adhesives.

Table 1. A brief list of wood types, properties, and adhesives commonly used for glulam window frames; refer to [36].

Plywood Type		Properties	Natural Durability	Maintenance	Adhesives
Solid wood	Picea spp.	Low density, light weight, moderate strength	Poor, need anticorrosive treatment	Check every year and recoat every 2–3 years	Phenol Formaldehyde Resin; Isocyanate Adhesive; Polyurethane Adhesive; Phenol-Urea-Formaldehyde Resin
	Pinus sylvestris	Low density, light weight, easy to process	Medium, requires antiseptic treatment	Scheduled maintenance and recoat every 2–3 years
	Larix	Medium density, high strength, good weather resistance	Good, but prone to cracking when dry	Check regularly, sand and recoat if necessary
	Quercus rubra	High density, high hardness, good wear resistance	better, needs to be treated with preservative	Annual maintenance inspection, regular sanding and repainting
	Tectona grandis	High density, High stability	excellent durability, resistance to insect erosion	Minimal maintenance, annual cleaning and application of protective oil or wax
Modified wood		Good dimensional stability and mechanical strength, not easy to crack, deformation	Strong weather resistance, good corrosion resistance, no need for conventional anti-corrosion treatment, Suitable for long-term outdoor exposure	Maintenance costs are low, but reasonable coating and regular maintenance can effectively extend the service life and beauty	demands special surface preparation or the use of special adhesives

provides a brief list of wood types, properties, and adhesives commonly used for glulam window frames. Furthermore, different methods of assembly during this process significantly impact the performance of the resulting integrated wood product [34,35].

The layered and porous structure of wood allows for the design and modification of the material at its own structural and/or compositional levels. Improvement strategies generally fall into three major categories: physical, chemical, and composite modifications. Chemical modifications can be further divided into two techniques: subtractive modification and additive modification, as illustrated in Figure 5.

• Physical modifications

Compression densification is an effective approach for enhancing the mechanical properties, including the hardness and strength of wood [37,38,39]. Huang et al. [40,41] introduced a novel method of inter-layer compression, providing a unique avenue for selective and partial densification of wood. This technique allows the formation of one or multiple controllable high-density layers along the thickness of the wood, with control over the position and thickness of the compression layers, while achieving improvements in mechanical performance through compression requires a high compression ratio, leading to elevated manufacturing costs. Heat treatment is a simple and effective strategy for improving the dimensional stability and durability of wood; unfortunately, heat treatment may result in the collapse of the wood structure, limiting the final use of heat-treated wood [42,43,44]. Typically, pre-treatment with chemical substances before heat treatment can effectively mitigate this issue. Gao et al. [45] demonstrated a method of enhancing the mechanical properties of heat-treated wood without compromising its dimensional stability through pre-treatment with chemical substances; the combination of impregnation and heat treatment results in superior mechanical and durability properties of the wood [46,47]. Coating technology is widely applied in various fields, and different types of coatings can be used to protect wood based on desired characteristics and applications; these coatings include preservatives (anti-fungal agents), flame retardants, waterproofing agents, fillers, and more [48,49,50]. In addition, Ma et al. [51] effectively isolated wood from water contact by constructing a superhydrophobic coating on the wood surface; this coating alleviates issues such as deformation, cracking, and decay caused by the wood’s strong moisture/water absorption capacity. Simultaneously, it imparts new functionalities to the modified wood, including waterproofing and self-cleaning properties.

• Chemical modifications

Subtractive chemical modification involves the partial or complete removal of certain components from wood (such as cellulose, hemicellulose, lignin, extractives, and other functional groups), resulting in the creation of new chemical or pore structures, which can give wood new functions. The removal of wood components introduces more nano-pores on the cell wall without disrupting the original cell structure of the wood [52,53,54,55,56]. Delignification is a common subtractive chemical modification strategy, involving the removal of lignin from natural wood through chemical treatment. Delignification retains the pore structure of natural wood and the cellulose nanofibers in the cell wall and can be used to produce “white wood” (also called “nano wood”). For example, delignified wood reflects most incident light and appears white, as well as possessing a lower thermal conductivity than natural woods [54,57]. Carbonization is another common subtractive chemical modification strategy that can remove wood components (such as cellulose, hemicellulose, lignin, and extractives) and convert the residual material into amorphous carbon; carbonized wood inherits the cell structure of natural wood while enhancing the wood’s light absorption properties and corrosion resistance [58,59].

The additive chemical modification involves the addition of various components (such as inorganic particles, polymers, metals, and metal–organic frameworks) or functional groups to wood, resulting in different structures and materials with additional properties and/or functionalities [58,60,61]. In situ polymerization and mineralization have been extensively explored, achieving structural changes in wood at the cell wall level by coating, impregnating, or filling polymers or inorganic components into the cell wall and/or cavities [62,63]. In addition to the inherent properties of the added components, the location of the additives and the relationship between additives and wood components play a crucial role in the modification. For instance, filling polymer or inorganic particles into the nano-pores or lumens of the cell wall can result in different mechanical, permeability, and flame-retardant properties; chemical bonds formed between the original wood components and the added components are expected to enhance the structural stability of the modified wood [56].

Furthermore, physical and chemical modifications can be combined to tailor the wood structure. For example, dense wood (or “super wood”) can be produced by partially delignifying the wood and then subjecting it to mechanical compression. Compared to dense wood produced solely through mechanical compression, this combined process results in a higher degree of compression (volume reduction of up to 80%) and significantly improved tensile strength [64].

Figure 5. Methods for improving wooden window frame materials (chemical modification adapted from [56]).

Figure 5. Methods for improving wooden window frame materials (chemical modification adapted from [56]).

• Discussion

The natural properties of wood often limit its use in doors and windows. Physical improvements typically involve various processing techniques to alter the structural characteristics of wood. Mechanical compression, heat treatment, and surface coating processes are common in physical improvements. In particular, coating technology is widely used in the field of wood, including the production of glulam, and attention should be paid to the maintenance of the coated surface. Through these techniques, the macrostructure, microstructure, and surface properties of wood can be adjusted to impart new performance and functionality. Delignification and carbonization are common methods in chemical improvements, while in situ polymerization and mineralization are employed for additive chemical improvements. In comparison to physical improvements, chemical improvements allow for the deliberate reduction in or addition of components to the natural wood structure, enhancing flexibility in improvement processes across multiple scales. Additionally, physical and chemical improvements can be combined to tailor wood properties, significantly expanding its application areas. Apart from enhancing the wood itself, the stability and safety of the wood in use can be greatly improved through rational forming methods using adhesive technologies.

3.2. 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 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]. Therefore, in the application and installation of polymer window frames, not only should the creep phenomenon under tensile, compressive, shear, and other stresses be avoided, but also the material strength issue should be considered. The installation and reinforcement method of anchor connection can be used to control the shape and size of the window frame, and the ends can be connected with metal connectors to enhance their mechanical properties.

• Additives

Additives are used in the production and utilization processes of polymers to improve the rheological behavior during polymer production or the performance of finished materials (physical properties, environmental stability). Additives are generally inert substances and can be classified based on their main functions, such as plasticizers [97], stabilizers [98], lubricants [99], impact modifiers [100], etc. The selection of additives for a given polymer is determined by the desired modification of the finished product. Typically, additives must meet certain requirements [69,101]:

(I)

Compatibility with the basis resin: non-toxicity, absence of coloring, chemical inertness and neutrality, heat and light stability, low water absorption, no influence on the stability of the polymer or its color.

(II)

Wettability: good distribution of powders in the polymer matrix or adhesion of the fibers to the basis polymer, uniformity of quality and grain size, low abrasive action.

• Discussion

Polymer materials exhibit various properties superior to traditional materials, offering not only structural but also functional applications with vast development prospects. However, polymers are prone to aging behaviors influenced by environmental factors, especially when used as structural materials, as they are not inherently robust. In this chapter, we present advanced composite materials based on thermoplastic polymers, elastomeric polymers, and thermosetting polymers. These advanced composites are reinforced with organic and/or inorganic fibers and formulated with various fillers, such as organic, mineral, and metallic components, resulting in higher performance. When subjected to loads, the matrix ensures the cohesion and orientation of the material, bearing the transfer of stress, while the introduction of reinforcing agents and fillers affects the interface properties with the matrix. Therefore, the type, shape, and proportion of the matrix, reinforcing agents, and fillers, as well as the quality of the interface and the adopted production processes, are critical parameters influencing the performance of composite materials. Careful consideration of these factors is necessary to achieve the desired properties.

3.3. Metal

• Selection of categories

The application of pure aluminum is limited; therefore, it is often combined with other elements such as silicon, magnesium, manganese, etc., to form alloys [102]. The selection of aluminum alloy grades for a given application depends on several factors, including the required strength, stiffness, weight, corrosion resistance, and cost. Aluminum alloys are typically classified based on their chemical composition, performance, and intended use. There are various classification systems, including the Aluminum Association (AA) designation system, Unified Numbering System (UNS), International Alloy Designation System (IADS), and Military Specification System (MIL-SPEC) [103]. Among these systems, the AA designation system is the most used.

Table 2. A series of aluminum alloys about cast aluminum and forged aluminum (adapted from [107]).

Summary of Cast Aluminum Alloy Series
Series	Main Alloying Element (s)	Main Characteristics	Example Applications
1xxx	Pure aluminum	High electrical and thermal conductivity; excellent corrosion resistance.	Big electrical rotors.
2xxx	Copper	High strength But low corrosion resistance.	Cylinder heads and pistons, housing and bearings
3xxx	Silicon, copper and/or magnesium	High strength and wear resistance; good corrosion resistance.	Motor parts, structural parts, and marine and aircraft castings.
4xxx	Silicon	Moderate strength; high ductility; good impact resistance	Bridge railing support castings, dental equipment, and cookware.
5xxx	Magnesium	Moderate to high strength; high corrosion resistance; good machinability; attractive appearance.	Architectural and ornamental castings and welded assemblies.
7xxx	Zinc	Good finish; good corrosion resistance; high strength through heat treatment.	Automotive parts and mining equipment.
8xxx	Tin	Low friction.	Bearing and bushing applications.
1xxx	Pure aluminum	High electrical and thermal conductivity; excellent corrosion resistance.	Electrical conductors and chemical processing equipment.
2xxx	Copper	High strength-to-weight ratio; low corrosion resistance.	Truck wheels and suspensions, aircraft fuselage and wings.
3xxx	Manganese	Moderate strength and good workability.	General sheet work, recreation vehicles, and electronics.
4xxx	Silicon	Low melting point and thermal expansion; high wear resistance.	Welding wire and brazing alloy, architectural applications, and forged engine pistons.
5xxx	Magnesium	Moderate to high strength; good weldability; good corrosion resistance.	Appliances, automotive parts, and marine components.
6xxx	Silicon and magnesium	Medium strength with good formability; weldability, machinability; corrosion resistance.	Structural applications, architectural extrusions, and recreational equipment.
7xxx	Zinc	Moderate to very high strength.	Airframe structures, mobile equipment, and high-stress parts.

provides the main alloying elements for both cast aluminum and forged aluminum series, along with their key characteristics and some example applications. Forged aluminum alloys are generally stronger and more ductile than cast aluminum alloys, often used in applications requiring high strength and good formability, especially in the 5xxx and 6xxx series alloys. Due to their excellent balance of strength, formability, corrosion resistance, and weldability, they are particularly well suited for architectural applications [104,105,106,107].

• Corrosion

To prevent the corrosion of aluminum alloys, surface treatments such as anodization [108], painting [109], or coating [110] can be applied, as shown in

Table 3. Coating technologies for metal window (adapted from [113]).

Technologies	Principle	Characteristics
anodization	In the electrolyte, aluminum alloy is used as the anode, and a current is applied to generate an oxide film on its surface.	Forms thin (5~20 μm) to thick (60~200 μm) oxide films, dense with high hardness, strong adhesion, and remarkable corrosion resistance.
chemical oxidation	Placing an aluminum alloy in an oxidation solution at a certain temperature allows aluminum atoms to undergo oxidation reactions in the solution, forming a dense oxide film on the surface	Simple operation, low cost, film thickness ranging from 0.5 to 4 μm.
spraying	The spray material is heated by a heat source to a molten or semi-molten state, then accelerated to impact and deposit on the substrate surface, where it cools and forms a coating	Preparation is versatile, widely applicable for spray materials such as metals, alloys, or non-metallic materials, with coatings exhibiting high bond strength and low porosity.
laser cladding technology	Using a high-energy laser beam to melt powder coatings onto the substrate surface, forming a dense cladding layer	Obtaining coatings with special protective properties, tightly bonded to the substrate, with controllable thickness of the cladding layer and minimal defects.
Nano-coating	Involves creating nanoscale films on the surface of aluminum alloys	The nano-coating is dense, uniform, and smooth, enhancing both corrosion resistance and wear resistance. It also meets the dimensional requirements of high-precision components.

. Anodization is a process that significantly enhances the corrosion resistance of aluminum alloys. Anodization involves passing a current through aluminum while immersing it in an electrolytic solution; this results in the formation of a thicker and more durable oxide layer on the surface of aluminum, providing enhanced corrosion protection [111]. Additionally, aluminum alloys can be alloyed with elements such as copper, magnesium, or zinc, further enhancing their corrosion resistance [102,112].

• Thermal

The heat transfer in window frames primarily occurs through the solid’s thermal conductivity, as well as convective and radiative heat transfer within the cavities [114]. Optimization of window frame geometry, material modifications, and cavity filling can effectively enhance the thermal performance of window frames [115,116,117]. Thermal break structures are often employed for insulation. The main feature of thermal break aluminum is the incorporation of one or multiple thermal insulation “breaks” made of insulating materials (typically plastics or thermoplastic materials) within the framework of aluminum alloy. This thermal break structure isolates the interior and exterior aluminum materials, reducing heat conduction and improving insulation performance [114,118].

• Laboratory tests

Accurate and reliable design formulas are a prerequisite for ensuring structural safety. Over the past few decades, scholars have undertaken numerous research projects involving experimental, numerical, and analytical studies on various structural aluminum alloys. Evangelia et al. [18] discussed the reported experimental, numerical, and analytical studies on different structural aluminum alloys, providing a comprehensive review of the research. They also explored the applicability of international design codes to structural aluminum alloys to address knowledge gaps. Fire safety is a major concern in the design of aluminum alloy structures [119,120]. In the event of a fire, the temperature of aluminum alloy materials can rapidly rise, significantly reducing material strength [121]. The performance of materials in a fire can be determined through experiments, including steady-state and transient tests [122,123]. In addition, digital simulation and analysis represented by finite elements have been widely used in the structural design of aluminum windows; computational modeling of any newly proposed section saves on all three fronts: cost, time, and effort. Such models can serve as data generation tools for developing a robust machine learning model that can ultimately eliminate the need for tedious trial-and-error design approaches; at the same time, it also provides a scientific method for the reasonable selection of materials and the optimal design of structures [124,125,126].

• Discussion

In this chapter, we make a simple classification of aluminum alloys and introduce the properties and applications of different aluminum alloy profiles. To improve the corrosion resistance and thermal insulation performance of aluminum alloy, the anodic oxidation process is widely used because of its excellent effect on improving the corrosion resistance of aluminum alloy; the broken bridge structure is usually used to improve the thermal performance of the aluminum frame, but it needs to be used in conjunction with thermal insulation materials. In addition, more and more design formulas/models for the safety of aluminum alloy structures have been proposed, which greatly improves the applicability of aluminum alloys in wind, fire, earthquake, and other environments.

3.4. Combination Window Frames

High-performance frames often explore composite material options; aluminum-clad wood is an architectural material used for doors and windows. It is a frame formed by mechanically combining thermal break aluminum alloy profiles and wood. The two materials are connected through polymer nylon components. Considering the different coefficients of expansion of wood and metal, this composite structure meets both structural safety and energy-saving requirements, providing people with a more comfortable experience [127]. Polymers can be grafted onto the aluminum surface through salinization and subsequent reaction with ethylene-containing compounds, thereby altering the chemical and physical properties of the interface. Ethylene-coated aluminum is another option. Several methods have been explored to achieve ethylene coating on aluminum. One method involves encapsulating aluminum pigments with ethylene imine reactive dyes, creating a hydrophilic surface with good dispersibility in water [128]. Another method utilizes a polyethylene aldehyde/graphene oxide nanocomposite coating to enhance the corrosion resistance of aluminum alloy by forming a uniform protective layer on the surface [129]. Additionally, aluminum alloys, when used as structural components, can be reinforced with Fiber-Reinforced Polymer (FRP) composite materials. FRP composite layers are applied to the surface of aluminum components, and each layer is oriented in a specific direction, which can significantly improve the strength and stiffness of aluminum structural components, improving its higher load-bearing capacity and better resistance to external forces such as wind, earthquakes, and other environmental factors [130,131,132]. However, it is necessary to pay attention to the possibility of the coating peeling due to improper surface treatment, environmental effects (climate, temperature change), and mechanical damage (impact and friction, daily use), and carry out regular inspection and maintenance.

3.5. Sustainability Assessment

Sustainability is commonly defined as a development or practice that does not diminish the long-term productivity of natural resource assets on which a country’s income and development depend; sustainable development encompasses several characteristics, such as energy efficiency, environmental friendliness, and recyclability [133]. Since the late 1990s, numerous studies have been conducted, primarily in the form of life cycle assessments (LCAs), to evaluate the environmental impacts of windows [134,135,136,137]. Figure 7 illustrates the life cycle of different window frame materials.

• Life cycle assessment

Due to well-managed forestry systems, wood is now considered a renewable material, and the processing and production of wood have minimal environmental impact; at the end of their service life, they can be reused for another product, land-filled, used as fuel, or degraded through recycling [138,139]. The quantity of existing aluminum resources is not a significant concern because aluminum is a fully recyclable material; aluminum recycling avoids processes that consume a substantial amount of energy and the use of other chemical materials, contributing to a reduction in greenhouse gas emissions [140]. However, aluminum production requires substantial energy, including bauxite mining, Bayer refining, aluminum smelting, refining, and metal processing [141,142,143]. Polymers are energy-intensive during synthesis, and their entire life cycle has environmental impacts. For example, PVC releases many toxic elements during its production, and they are usually disposed of through land-filling or incineration that may leach additives like phthalates and heavy metals, potentially contaminating groundwater. PVC is a major source of chlorine in incinerators, contributing to the formation of the highly toxic synthetic chemical dioxin [144]. The recycling of polymer products involves categorizing waste as general material, and the quality of recycled material depends heavily on the impurity content, including other polymers or additives, making recycling technically and economically challenging [145]. Additionally, some cradle-to-gate analyses found that aluminum-clad wood windows and wood windows have lower environmental impacts compared to aluminum and PVC window frames. Wood has a low impact on the environment, with values for embodied energy, GWP, and acidification potential (AP) being the lowest, even when excluding carbon sequestration potential. Aluminum-clad wood windows, aside from increasing the environmental impact mainly due to the production of the aluminum cladding, share similar environmental characteristics with wood windows and can decompose into their basic components, aluminum and wood, at the end of their service life [146,147].

• Thermal performance

The significant variation in the thermal performance of windows is largely attributed to their frames having a higher thermal transmittance; this is especially pronounced in buildings with a high window-to-wall ratio, where windows exhibit higher thermal transmittance than the opaque parts of the wall [148]. A study by Lawson shows that the global warming potential (GWP) of wood, PVC, and aluminum window frames is 1:11:26, respectively [149]. Furthermore, Gustavsen et al. [150] conducted a market review of commonly used high-performance window frames in Europe, including wood, aluminum, PVC, and aluminum-clad wood materials. They presented 56 different window frames with various geometric shapes, materials, and styles. The study highlighted the incorporation of common high-performance insulation materials, such as rigid polyurethane (PUR), polyethylene (PE), and extruded polystyrene (XPS), into the frame cavities. The U-value (or U-factor), a measure of thermal transmittance, ranged from 0.63 W/m² K to 1.14 W/m² K. Improving the thermal performance of window frames will increase their ecological impact from cradle to gate.

In general fires, the combustion deformation of doors and windows in buildings poses a more serious threat to life than the combustion of building structures. According to GB/T 8624-2006 [151] Classification of Burning Performance of Building Materials and Products, the average temperature of a general building fire is between 700 and 900 °C, and wood is usually carbonized at a rate of 0.64 mm per minute. The carbonization layer naturally isolates the wood from the outside world and increases the temperature that the wooden components can withstand. If the aluminum-clad wooden window frame is exposed to an open flame at 800 °C for 30 min, only 19mm will be carbonized. The integrated materials in the aluminum-clad wooden window have undergone certain flame-retardant treatment during the processing, which plays a better role in fire prevention and disaster reduction.

• Life expectancy

Durability or lifespan depends on the use of appropriate products, correct installation, and regular maintenance [152,153,154]. During the installation of window frames, efforts should be made to avoid damaging the original building structure and to pay attention to whether the installation process complies with regulations. In this sense, knowledge regarding the service life of window frames is extremely relevant, aiding the adoption of adequate solutions in the design and maintenance stages. In terms of window frame life prediction, the service life prediction methods are divided into three main groups [155]: deterministic (e.g., factor and graphical method); probabilistic (e.g., Markov chains); and engineering methods (e.g., failure modes effects analysis (FMEA)). Combined with the actual case of Portugal, Fernandes et al. [156] analyzed 173 samples of aluminum window frames and 41 samples of wood window frames and proposed a prediction model for the service life of aluminum window frames and wood window frames based on the factor method, which added additional value to the revision of the ISO 15686-1 [157] standard. Banaś et al. [137] evaluated and analyzed the degradation of 182 window frames (including aluminum, wood, and PVC) under real-use conditions and environmental exposure, and established a window frame degradation model, which reflected the evolution law of window frame degradation with time. In a simplified approach, the standard for durability and the different European EPD operators [136,156] specify that window frames must present an estimated service life of at least 30 years.

In installation and maintenance, at present, the installation grade requirements of building exterior windows are mainly based on the current national standard GB/T 7106 [158] in classifying and testing their air tightness, water tightness, and wind pressure resistance. Asif et al. [135] conducted a study indicating that aluminum alloy window frames, apart from being cleaned for aesthetic reasons, have lower maintenance costs. Coatings, anodization, and other treatments can extend their lifespan. Specifically, anodized aluminum alloys can endure for over 20 years in harsh environments. On the other hand, PVC window frames encounter issues in maintenance primarily due to their surface vulnerability; scratches caused by hard particles can affect their appearance and durability. Recommended maintenance methods for PVC frames include using non-alkaline cleaners for regular cleaning, suggested every six months, as repair are difficult once the frame is damaged. Maintenance costs for wooden window frames are relatively high, requiring periodic coatings to maintain appearance and provide resistance against weathering and degradation. Typically, exterior painting is recommended every 5 years and staining every 3 years. Aluminum-clad wooden frames generally cost 14–18% more than pure wooden frames, but they incur lower external maintenance costs due to the stability provided by the aluminum cladding, which lasts for an extended period.

4. Conclusions

Improvements to windows can directly reduce their environmental impact. As a key element, window frames not only ensure the structural integrity of the window system but also play a significant role in energy efficiency and environmental conservation. In GB/T 50176-1993 [159], it is explicitly stipulated that buildings in severely cold areas and cold regions should use doors and windows with good insulation performance, such as wooden windows, plastic windows, aluminum–wood composite doors and windows, and aluminum–plastic composite doors and windows. However, in the process of selecting and using window frames, the performance of window frames is often influenced by the choice of materials, structural design, and the operating environment. This review examines commonly used window frame materials in the market, including wood, polymers, and metals. It covers the performance of different window frame materials, structural design considerations, and issues encountered during use. By considering existing improvement solutions, the review outlines the challenges that future research in this field may face. The specific conclusions are as follows:

• Wood, as an environmentally friendly material, has minimal environmental impact throughout its life cycle. The dimensional stability of wood when exposed to water, temperature, bacteria, or weather for extended periods is a crucial consideration, given the hydrophilic and biodegradable nature of wood cellulose. Heat treatment improves wood stability against water and bacteria, but it may compromise mechanical properties due to a significant reduction in hydrogen bonds. Coating wood’s outer or inner surfaces with stable polymers or inorganic materials as a stopgap measure can enhance stability without sacrificing mechanical performance. Moreover, current improvements mostly remain at the laboratory scale. Chemical enhancements to small-sized veneers (thin wood pieces, sometimes bark) and assembling them into large panels are promising directions to meet the dimensional requirements of the final product. At the current stage of using wood as a window frame material, it needs to be processed into engineered wood products. Whether focusing on the improvement of wood itself or the design of wood components, narrowing the gap between academic research and industrial practices is an essential area for future research. This will facilitate the practical application of more wood materials for the benefit of society.
• Polymer materials are systems composed of a relatively large number of components; hence, the actual number of composite materials that can be realized from this array of basic elements is virtually infinite. Thermosetting composite materials exhibit excellent mechanical properties and thermal resistance at high temperatures. Composite materials reinforced with glass fibers and carbon fibers demonstrate outstanding tensile and compressive strength. Advanced composite materials are currently under research and application in various industries, such as glass fiber-reinforced polyurethane window frames. However, polymers are energy-intensive during the synthesis process and are associated with the generation of toxic gases. This impact on the environment persists throughout the entire life cycle of polymers. The heterogeneity of polymers and their difficulty in degradation make recycling both technically and economically challenging.
• Aluminum alloy, known for its excellent mechanical properties and recyclability, has found increasing applications in structural engineering. This trend has prompted extensive research on the structural performance of aluminum alloys and the development of precise design methods. The production of aluminum alloys falls within the category of energy-intensive industries. The use of thermal break structures is common to enhance the thermal performance of aluminum alloy window frames. However, this often requires collaboration with insulation materials to achieve optimal thermal properties. Improving the thermal performance of window frames may increase their ecological footprint from cradle to gate, and there is currently no conclusive evidence on how much the reduction in heating, cooling, and power loads during the usage phase can offset this environmental impact.
• Combination window frames represent a compromise by mechanically combining the advantages of different materials. Compared to using a single type of window frame material, combination window frames, which involve the stacking of different materials, reduce the overall environmental impact. They demonstrate higher structural safety and energy efficiency performance, providing enhanced comfort experiences. This approach holds promising market prospects. The future selection of window frame materials is diverse, including wood, aluminum alloy, polymer, and more. Each material has its unique characteristics and advantages, so the choice of window frame material requires consideration of various factors such as durability, maintenance costs, insulation performance, and environmental friendliness. In the future, with technological advancements and increasing emphasis on environmental protection and energy efficiency, window frame materials are expected to prioritize ecological performance and energy efficiency, and there will also be a greater focus on the sustainability and recyclability of materials.