Recent Advances in Biocomposite Materials Reinforced with Raw or Minimally Processed Wool: Fabrication Methods, Properties and Applications—A Systematic Review

Abstract

Sheep wool is a keratin-based natural fiber increasingly explored as a low-impact reinforcement and multifunctional modifier in composites, enabling valorization of coarse or waste wool streams. This systematic review consolidates evidence on raw or minimally processed wool-reinforced composites across polymer matrices and mineral binders. Following a registered protocol and Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020, Scopus and Web of Science were searched for English-language journal articles (2015–2025), yielding 44 included studies after screening. Evidence mapping shows polymers dominate (33/44; thermosets 19/44), while mineral binders account for 11/44. Wool is mainly used as short fibers (27/44), with woven (9/44) and nonwoven/felt (8/44) architectures appearing in laminates and insulation products. Because heterogeneity limits pooled meta-analysis, outcomes are synthesized using matched-control comparisons where available (27/44) and interpreted with a TRiC appraisal (Transparency, Reproducibility, and Credibility). Mechanical effects are highly conditional: gains in impact/energy absorption and occasional tensile/flexural stress improvements coexist with frequent losses linked to dispersion, wetting/impregnation and void sensitivity. Functional trends are similarly system-dependent, with promising but uneven evidence for acoustic performance, variable thermal conductivity shifts, and formulation-driven fire behavior. Moisture uptake and durability emerge as principal translation bottlenecks, motivating minimum reporting and design practices to improve comparability and application readiness.

1. Introduction

Natural-fiber composites are increasingly under investigation as lower-impact alternatives to conventional composite systems, particularly where weight reduction and multifunctionality are required alongside sustainability goals. Reviews of natural-fiber-reinforced polymers consistently emphasize that the main technical barriers to robust, transferable performance are interface quality, moisture sensitivity, and fire behavior, which often dominate performance variability more than the nominal fiber type [1,2].

Within this broader field, sheep wool (a keratin fiber) is a distinctive reinforcement candidate. Wool has been repeatedly highlighted for its intrinsic crimp and resilience, as well as for multifunctional potential (e.g., thermal/acoustic behavior and comparatively favorable flammability features associated with keratin chemistry), making it attractive for applications beyond traditional textiles [3,4,5]. In parallel, the recent literature on building materials frames coarse or waste wool as an underutilized stream with clear circular-economy relevance, motivating valorization routes into construction products and composite components rather than apparel/textiles [6,7].

However, the term “wool-reinforced composites” does not describe a single material class. The literature spans fundamentally different matrix families (thermosets, thermoplastics, and bio-based polymers, as well as cementitious/gypsum/earthen and related mineral binders), reinforcement architectures (predominantly short fibers, with more limited woven/nonwoven formats), and manufacturing routes (melt compounding/injection molding, lay-up/infusion, casting, and cementitious mixing/casting). This diversity has two consequences: (i) it makes direct cross-study comparison difficult, and (ii) it encourages fragmented reviews that focus on either polymer systems or construction/mineral systems in isolation. Recent wool-focused reviews reflect this split—for example, polymer-oriented syntheses consolidate wool–polymer strategies and interface approaches, while construction-oriented reviews emphasize wool in cement-based and building contexts, including durability constraints in alkaline matrices [3,4,6].

What this paper adds. To support design-relevant interpretation across the full field, this study provides a materials-focused systematic review that deliberately unifies polymer-matrix and mineral-binder wool composites within a single comparative framework, while restricting scope to raw or minimally processed wool fibers acting as fibrous reinforcement. The review follows PRISMA 2020 and a pre-defined protocol, using reproducible database searches (Scopus and Web of Science), duplicate handling and screening (Rayyan), and full-text exclusions tailored to keep the corpus within engineering/materials applications (excluding biomedical-only and textile-only domains, among other criteria) [8]. The final inclusion set comprises 44 journal articles (2015–2025).

Because heterogeneity in formulations, processing routes and test conditions limits the defensibility of a single pooled meta-analysis, this study combines evidence mapping with comparative synthesis where appropriate controls/baselines are reported, and it appraises reporting quality and interpretability using the TRiC framework (Transparency, Reproducibility, and Credibility) to guide how strongly findings can be generalized.

Research questions. The synthesis is structured around the following research questions:

RQ1 (Design space): What matrix families, wool forms (short fibers vs. woven/nonwoven), and interface strategies dominate the 2015–2025 literature?

RQ2 (Processing): Which fabrication routes are most commonly used for each matrix family, and which processing variables are most strongly linked to dispersion/impregnation and porosity/void sensitivity?

RQ3 (Mechanical performance): Under what conditions does wool reinforcement improve—or degrade—tensile, flexural, compressive, and impact performance relative to comparable controls?

RQ4 (Functional performance and trade-offs): What trends and recurring trade-offs emerge for thermal, acoustic, fire-related and moisture/durability-related properties?

RQ5 (Application fit and maturity): Which applications are most frequently targeted, how mature is the evidence base (standards, conditioning, comparable controls), and what barriers most constrain translation?

RQ6 (Reporting priorities): What minimum reporting and experimental-design practices would most improve reproducibility and enable stronger cross-study synthesis?

Paper structure. Figure 1 summarizes pathways from coarse/minimally processed wool to reinforcement in polymer and mineral matrices. Section 2 describes the protocol, eligibility criteria, search strategy and screening workflow. Section 3 reports the evidence map and comparative synthesis across material systems, fabrication routes, and property categories. Section 4 discusses cross-cutting mechanisms and translation bottlenecks (e.g., microstructure/process sensitivity, moisture durability, and formulation-dependent fire behavior), interpreted in light of TRiC. Section 5 provides concise conclusions and actionable research priorities. Abbreviations used in the manuscript are listed in the Abbreviations Section.

2. Methodology

2.1. Protocol and Reporting Standard

This article presents a materials-focused review of wool-reinforced biocomposites, supported by a transparent and reproducible literature identification and selection workflow. The final database searches were executed on 11 January 2026. The search, screening, and reporting of records followed the PRISMA 2020 guidelines. A review protocol defining the scope, eligibility criteria, and data-extraction plan was established prior to screening and deposited on the Open Science Framework (OSF: https://osf.io/nfxpe/) (accessed on 13 February 2026). The OSF repository provides the full set of selection and filtering criteria, operational definitions, and detailed screening documentation (including complete database-specific search strings and screening logs), ensuring transparency and reproducibility.

2.2. Eligibility Criteria

Studies were eligible if they reported a matrix-based composite system comprising (i) an identifiable matrix phase (polymeric/bio-based polymeric matrices or mineral binders such as cementitious, gypsum, mortar/concrete, geopolymers), and (ii) wool present as a fibrous reinforcement (i.e., fibers acting as reinforcement/filler in the composite). Searches were restricted to English-language journal articles published between 1 January 2015 and 31 December 2025. Records primarily related to biomedical/clinical topics and textile/apparel contexts were additionally filtered out at the query level to reduce off-scope retrieval. In this review, “minimally processed wool” was operationally defined as wool that retains a fibrous morphology and a reinforcement/functional role within an identifiable matrix (e.g., scoured/washed, dried, chopped/carded, and/or surface-modified by .e.g., alkali or silane treatments). In practice, this category includes typical pre-conditioning steps (washing/scouring, drying and basic mechanical size conditioning), with mild alkali/silane surface treatments reported in a subset of studies.

Full-text exclusions were applied using the following criteria:

• E1—Wrong reinforcement: Wool is not used as a fibrous reinforcement in the composite (e.g., “wool” mentioned generically, non-wool fibers only, or wool not acting in a reinforcing role).
• E2—Biomedical-only: Studies primarily focused on biomedical/clinical applications (e.g., tissue engineering, scaffolds, implants, wound healing/dressings, drug delivery, bioinks/hydrogels, biosensors) were excluded as outside the engineering/materials scope.
• E3—Out-of-scope matrix: The matrix is clearly outside scope or cannot be identified as part of a matrix-based composite system from the full text.
• E4—Review/non-original: Non-original publications (reviews, perspectives, editorials, letters, commentaries) were excluded.
• E5—Out-of-scope application: Records outside the engineering/materials scope were excluded when they did not describe a wool-fiber-reinforced composite with identifiable fabrication details and performance outcomes relevant to the review objectives.
• E6—Outside the time window: Records were excluded when the publication date fell outside 1 January 2015–31 December 2025.
• E7—Retracted/withdrawn: Retracted or withdrawn publications were excluded.

The final inclusion set comprised 44 journal articles (the full list is provided in Appendix A).

2.3. Information Sources

Bibliographic searches were conducted in Scopus (Elsevier) and the Web of Science Core Collection (Clarivate). The complete database-specific search strings and applied database filters are provided in the OSF repository “https://osf.io/nfxpe/ (accessed on 13 February 2026)”. In addition, backward and forward citation screening of key included articles was performed to identify any further eligible studies.

2.4. Search Strategy

The search strategy was designed around four concept blocks combined with AND: (i) wool fibers, (ii) composite context, (iii) reinforcement role, and (iv) matrix/binder families (polymers and mineral binders). To reduce off-scope retrieval, two negative keyword blocks were applied to exclude predominantly biomedical/clinical topics and textile/apparel contexts. Database-specific syntaxes were used (TITLE-ABS-KEY in Scopus; TS in Web of Science).

2.5. Study Selection Process

All retrieved records were imported into Rayyan for duplicate removal and screening. Duplicate detection in Rayyan identified 118 duplicate instances, and 59 duplicated references were removed. When duplicate pairs were detected, the record with the most complete metadata (e.g., abstract, keywords, DOI) was retained. Screening was performed in two stages (title/abstract, then full text) using the predefined exclusion criteria (E1–E7).

The overall identification and selection process is summarized in the PRISMA flow diagram in Figure 2.

2.6. Data Extraction and Coding

A standardized extraction form was used to collect and code data from all included studies (n = 44). For each article, we extracted information describing: (i) the material system (matrix type/family, wool reinforcement form, fiber treatment or interfacial strategy, reinforcement fraction reported as wt% and/or vol%, hybridization with other reinforcements or additives); (ii) the fabrication route (e.g., compression molding/hot pressing, injection molding, casting/lamination/infusion, cementitious mixing/casting) and key processing parameters when reported; (iii) testing protocols (standards, specimen conditioning and test settings when available); (iv) performance outcomes (mechanical and functional properties); and (v) application domain and any maturity indicators reported. When multiple formulations were reported within the same article, each formulation was recorded as an individual data point while the study contributed once to the evidence-map counts.

To improve cross-study comparability, property values were harmonized to consistent units whenever feasible, and reinforcement fraction units were recorded as reported (wt%/vol%), noting cases where conversions were not possible due to missing density information. If a required data item was not explicitly reported, it was coded as not reported (NR). The full extracted dataset and coding scheme are provided in Supplementary Materials (Table S1).

These extracted variables were subsequently used to build the evidence map and to structure the comparative synthesis by matrix family, processing route, and property category, supporting the analysis of material systems, processing strategies, performance outcomes, application domains, and reporting gaps (RQ1–RQ6).

2.7. Study Reporting and Interpretability Appraisal (TRiC)

To appraise reporting quality and study interpretability across the included studies (n = 44), we applied a structured rubric termed TRiC (Transparency, Reproducibility, and Credibility), adapted from LeBel et al.’s credibility framework [9]. In this review, TRiC is used as an engineering-oriented appraisal of reporting completeness and interpretability, rather than as a formal clinical-style risk-of-bias tool.

Transparency (T) captures how completely studies report the information required to interpret their methods and outcomes (materials: matrix family, wool form and fraction; processing parameters; test standards/conditions; and data presentation). Reproducibility (R) evaluates whether the reported formulation, fabrication route, and testing/conditioning details would allow replication in principle. Credibility (C) evaluates whether comparative claims are supported by an appropriate control/baseline, whether replication/uncertainty reporting is provided where relevant, and whether conclusions are internally consistent with the presented results.

Each TRiC dimension was rated at three levels (High/Medium/Low) using predefined anchors (Appendix B). Two reviewers independently applied the TRiC criteria to all included studies based on the full text and any supplementary files; disagreements were resolved by discussion and, when necessary, with the involvement of a third reviewer. TRiC ratings were not used to exclude studies; instead, they were used to contextualize confidence in cross-study interpretation and to highlight reporting gaps that limit comparability. The full TRiC rubric (dimension anchors) and the resulting study-level ratings with the main limitations are reported in Appendix B.

2.8. Data Synthesis and Analysis Approach

Data were synthesized to address the research questions through a combination of evidence mapping, comparative descriptive analysis, and narrative technical synthesis. First, an evidence map was generated from all included studies (n = 44) to summarize distributions across matrix families, wool reinforcement forms, processing routes, reported property categories, and application domains.

Second, where studies reported comparable baselines or controls, outcomes were summarized using cross-study descriptive statistics (study counts and proportions; ranges and central tendency where feasible). Given the heterogeneity in material systems, processing parameters, and testing standards, a formal meta-analysis was not pursued. Instead, comparisons were interpreted in light of reported microstructural features (e.g., fiber dispersion, porosity, and interfacial observations) and methodological transparency as assessed by TRiC.

Finally, results were organized into technical synthesis blocks aligned with the review scope: (i) reinforcement and fiber-related aspects (wool morphology, treatments and reinforcement architectures), (ii) composite system definition (matrix families and formulations), (iii) fabrication methods and application context, and (iv) performance outcomes, including both mechanical and functional properties.

3. Results

This section synthesizes the included literature (n = 44) to provide (i) an evidence-map overview of material systems and research focus, and (ii) a comparative technical synthesis of fabrication approaches, mechanical performance, functional performance, and application domains.

3.1. Authors and Keyword Analysis

Figure 3 represents the authorship analysis, performed by extracting author names from bibliographic records and standardizing them to a consistent convention to minimize fragmentation due to punctuation, initials, or alternative spellings. Author prominence was then evaluated through frequency of occurrence across the corpus, which identifies recurrent contributors and the research teams most consistently represented in the retrieved literature.

Within this framework, the most prolific and repeatedly appearing authors are Rayed Alyousef, Alessandro P. Fantilli, and Daria Jóźwiak-Niedźwiedzka, indicating that these researchers and their associated groups constitute central reference points in the dataset. A second group of authors also shows strong recurrence, most notably Hisham Alabduljabbar and Hossein Mohammadhosseini. Beyond these leading names, the authorship landscape is characterized by a broad tail of contributors that occur less frequently, which is typical of applied and interdisciplinary research domains where publications often reflect project-based collaborations and institutionally clustered teams rather than a single, densely interconnected community.

A year-by-year view (covering 2015–2025) reveals a structured evolution in the composition of collaborations. The early 2020s are dominated by more recent active groups, particularly those centered on Alyousef and Alabduljabbar, indicating a shift in the core contributing teams over time. At the most recent end of the time window, authors such as Yohanes Abebe and Sivaprakasam Palani appear primarily in the latest years, consistent with emerging or currently expanding collaborations. As shown in Figure 3, the temporal structure suggests a transition from an earlier, compact co-authorship cluster toward newer teams that become increasingly prominent in the 2022–2025 period, reflecting a field that is both cumulative and dynamic.

Keyword analysis was carried out by extracting terms from the bibliographic records and standardizing them to a consistent format to reduce fragmentation. The most recurrent terms highlight a clear thematic core focused on the “mechanical properties” of wool-based composite materials, with “wool” acting as the main hub connecting multiple application- and method-oriented topics. Among the most repeated keywords, “mechanical properties”, “wool”, and “thermal conductivity” dominate the landscape, followed by recurrent materials/structure terms such as “biocomposites”, “hybrid composite”, “composites”, and “microstructure”, followed by application-oriented terms including “sound absorption” and “insulation”. The network organization reveals three main thematic groupings, as shown in Figure 4.

The map shows three main clusters. The central one links mechanical properties, wool, and composites, extending toward flexural behavior, PLA, flax, and sustainability—pointing to a core line on the mechanical performance of wool-based (bio)composites in bio-based/sustainability frames. A left-side cluster groups thermal conductivity, sound absorption, and insulation with sheep wool fibers and microstructure, highlighting wool’s role as a porous/functional medium for thermal and acoustic insulation where microstructure is tied to outcomes. A right-side cluster connects polymer composites and natural fibers with FTIR/TGA and related terms, reflecting a broader characterization stream that intersects the wool core via bridging keywords.

The temporal overlay suggests a shift from earlier general descriptors and characterization toward more recent emphasis on performance and formulation topics (e.g., composites, flexural properties, polymer composites), while “mechanical properties” remains consistently central. Overall, the field appears to be cumulative yet evolving, with stable foundations alongside recent diversification in materials, testing/characterization routes, and application targets.

3.2. Evidence Map of the Literature (2015–2025; n = 44)

Building on the evidence-map distribution (Figure 5), the included studies can be organized into recurring material–process families.

The literature is dominated by polymer-based composites (33/44), with thermosets as the most common matrix family (19/44), followed by thermoplastics (9/44). Mineral-binder systems (11/44) form the second major stream, largely associated with construction materials (mortars, concretes, gypsum-based composites, and earthen/clay-derived products). Wool is most frequently implemented as short fibers (27/44), whereas woven textiles (9/44) and nonwoven/felt structures (8/44) appear mainly in laminate-oriented polymer composites and in insulation-oriented products, respectively. In terms of processing, hand lay-up/lamination and infusion-type routes are the most represented (16/44), consistent with hybrid laminate architectures; cement/gypsum mixing–casting accounts for 11/44; compression molding/hot pressing for 9/44; and melt-based processing (extrusion/injection/thermoforming) for 4/44, with the remaining studies using rubber compounding/vulcanization or being classified as other/unclear.

Beyond system types, the evidence map highlights reporting and outcome coverage, which determines how far cross-study comparisons can be taken. Most studies report at least one recognized test standard (42/44), and a comparable baseline/control is available in 27/44 studies. Regarding reported outcomes, mechanical properties are provided in 39/44 studies and functional performance metrics (e.g., thermal, acoustic, fire, moisture/aging) in 34/44 (

Table 1. Reporting and outcome coverage across included studies (n = 44).

Indicator	Operational Definition (How It Was Counted)	n/44	%
Standards reported	Study explicitly reports at least one test standard (e.g., ASTM/ISO/EN) for any measured property	42	95.5
Comparable control available	Study includes a control/baseline that is directly comparable to the composite (e.g., neat matrix or reference mix tested under the same conditions)	27	61.4
Control reported (any)	Study reports some form of control/baseline (comparable or not)	28	63.6
Mechanical outcomes reported	Study reports at least one mechanical property (e.g., tensile/flexural/compressive/impact, etc.)	39	88.6
Functional outcomes reported	Study reports at least one functional property (e.g., thermal/acoustic/fire/moisture/aging, etc.)	34	77.3

Note: Counts are at the study level (one count per paper). “Functional outcomes” includes thermal, acoustic, fire/flame-retardancy, moisture/water absorption/aging, and related non-mechanical performance metrics.

). This combination—high standards of reporting but uneven availability of comparable controls—supports descriptive and comparative synthesis, whereas formal meta-analysis remains constrained by heterogeneity in formulations, processing and test conditions.

Declared application targets concentrate around construction (13 study-level mentions) and automotive-related applications (7), while insulation (general), insulation (thermal) and packaging each appear in two studies; additional niche targets (e.g., crashworthiness- or energy-absorption-oriented demonstrators, and acoustic/sound-absorbing construction products) appear as single-study mentions. These distributions motivate the structure of Section 3.3, Section 3.4, Section 3.5 and Section 3.6: we first synthesize material systems and processing strategies (Section 3.3), then consolidate mechanical and functional performance trends (Section 3.4 and Section 3.5), and finally map application maturity and reporting gaps that constrain translation (Section 3.6). The following sections synthesize these patterns by matrix family, highlighting processing levers and typical performance outcomes.

3.3. Materials and Fabrication Routes

Table 2. (a). Typical wool-based polymer-matrix systems and manufacturing routes (2015–2025). (b). Typical wool-based mineral-binder systems and manufacturing routes (2015–2025).

Matrix Family (Typical Matrices)	Typical Wool Reinforcement Form	Typical Manufacturing Route(s)	Main Targets (Typical Products)	Representative Studies
(a)
Thermosets (epoxy, unsaturated polyester; incl. bio-epoxy)	Short fibers; woven fabrics/laminates; hybrid stacks (wool–plant/wool–glass)	Hand lay-up/lamination; vacuum infusion/VARTM; casting/lamination	Structural laminates; crash/energy-absorption tubes; mechanical performance demonstrators	[10,11,12,13,14,15]
Thermoplastics (PLA, PP, recycled plastics)	Short (often recycled) fibers; occasionally nonwoven/felt for panels	Melt compounding (extrusion/internal mixing) + injection molding; hot press/thermoforming	Injection-molded parts; packaging/consumer products; lightweight components	[16,17,18,19,20,21]
Elastomers (natural rubber; rubber blends)	Short/coarse fibers	Rubber compounding + vulcanization/press curing	Green rubber composites; damping/toughness-oriented components	[22,23,24]
Biopolymers/proteins (wheat gluten, soy protein; plasticized)	Short fibers dispersed; porous panels (freeze-dried) in selected cases	Hot pressing/compression molding; aqueous mixing + freezing/freeze-drying	Acoustic/thermal panels; fire-safe biopolymer composites; packaging candidates	[25,26]
(b)
Cementitious binders (cement mortars, concretes; incl. SCMs/additives)	Short fibers (mm–cm)	Mixing + casting; standard curing regimes	Mortars/concretes for construction; mechanical/transport and durability-related studies; sound-absorbing mixes	[27,28,29,30,31]
Gypsum binders (gypsum plaster composites, tiles/boards)	Short fibers; hybrid fiber blends in selected cases	Mixing + casting/molding; panel/tile forming	Gypsum boards/ceiling tiles; acoustic/thermal performance products	[32,33,34]
Earthen/clay-based systems (stabilized earth blocks, unfired bricks, clay–cork blends)	Short fibers (often cm scale)	Mixing + pressing (blocks/bricks) + curing	Unfired bricks; stabilized earth blocks; lightweight building units	[35,36,37]

a,b provide an overview of the main material–process families and fabrication routes reported for wool-containing composites, separated into polymer-matrix systems (Table 2a) and mineral-binder systems (Table 2b).

Polymer-matrix composites dominate the corpus and are mainly represented by thermosets and thermoplastics, with smaller clusters in elastomers and protein/biopolymer matrices. Thermoset systems (typically epoxy and unsaturated polyester) introduce wool as short fibers or woven textiles (often in hybrid laminates), and are mostly manufactured via hand lay-up/lamination and infusion-type routes. Accordingly, impregnation quality, void sensitivity and fiber conditioning (washing/scouring and drying) are key for interpreting mechanical outcomes, with interface treatments or stacking-sequence design reported in a subset of studies.

In the reviewed studies, “raw/minimally processed” wool is rarely incorporated as shorn. The most recurrent preparation steps reported are (i) scouring/washing (often with rinsing) to remove grease/dirt and reduce variability, (ii) drying to limit free moisture prior to melt compounding or resin impregnation, and (iii) mechanical conditioning such as opening/carding and cutting/chopping to target fiber lengths (mm–cm). In line with the eligibility definition used in this review, some studies additionally report mild surface modification (e.g., alkaline or silane-based treatments) as an optional strategy to improve wetting/adhesion or tailor performance, but intensive functionalization is not a prerequisite within the present scope. These preparatory steps are highlighted because they strongly affect dispersion/impregnation, void formation and interfacial quality, and therefore help to explain the observed scatter in mechanical outcomes.

Thermoplastic systems (e.g., PLA, PP and recycled plastics) primarily use short fibers processed by melt compounding followed by injection molding or hot pressing/thermoforming. Here, dispersion, fiber attrition and moisture management during compounding govern porosity and interfacial quality, and many studies additionally report thermal/fire-related behavior or degradation. Elastomeric matrices (e.g., natural rubber) rely on rubber compounding and vulcanization, where performance depends strongly on fiber distribution and fiber–rubber interlocking. Protein/biopolymer matrices (e.g., soy protein and wheat gluten) generally use short fibers and are formed by hot pressing/compression molding, often targeting acoustic/thermal functionality.

Mineral-binder composites form the second major stream and are closely tied to construction applications. In cementitious binders (mortars and concretes), wool is typically added as short fibers through mixing and casting, and performance reflects the interaction between fiber addition, mix design, curing and resulting porosity/workability; hence, mechanical results are frequently discussed alongside moisture/transport indicators. Gypsum-based systems follow similar mixing–casting/panel-forming routes, often aiming at lightweight boards or tiles with acoustic/thermal functionality. Earthen/clay-based systems (e.g., stabilized earth blocks or unfired bricks) combine wool short fibers with pressing and curing, where stabilizers and moisture control strongly condition thermo-mechanical response.

Across both streams, outcomes are governed by a small set of recurring design levers: wool architecture (short fibers vs. textiles/felts), processing route (dispersion/wetting and porosity/void sensitivity), and reporting choices (wt% vs. vol%, standards and availability of comparable controls). These factors guide the next sections, where mechanical trends (Section 3.4) and functional performance (Section 3.5) are synthesized in relation to the underlying material–process families.

3.4. Mechanical Performance

Mechanical evidence is substantial but uneven across matrix families and test types. As shown in Figure 6, polymer matrices dominate mechanical testing, particularly thermosets, which most frequently report tensile and flexural properties and also contribute the majority of impact/energy-absorption datasets. Thermoplastics provide additional tensile and flexural evidence but fewer compressive and impact results. In contrast, mineral binders concentrate on flexural and compressive stress at failure, while impact-related testing is comparatively scarce; other matrix families (biopolymers/proteins, elastomers, and “other”) remain sparsely represented. This coverage pattern constrains broad generalization and supports a controlled-comparison synthesis whenever matched controls are reported. The underlying extracted dataset used to generate the evidence map and relative change (Δ%) synthesis is provided in Supplementary Materials (Table S1).

Given this uneven test coverage across matrix families and the resulting constraints on broad generalization, Figure 7 synthesizes the evidence on mechanical performance by summarizing the relative change (Δ%) of wool-containing formulations compared with their matched controls across key mechanical properties. Accordingly, Δ% values are interpreted as within-study comparative signals rather than fully harmonized effect sizes across studies. A “matched control” was defined as a control formulation reported within the same study and intended by the authors as the direct comparator to the wool-containing formulation under the same test setting. Where reporting allowed, comparability was checked against the test method/standard, specimen geometry, conditioning and curing; when such details were missing or not clearly aligned, the datapoint was retained for evidence mapping but interpreted cautiously and explicitly discussed as a limitation. By reporting the median, interquartile range, and the overall spread of observed outcomes, the box-and-whisker representation enables a consistent cross-study comparison of both typical trends and variability in the reported mechanical effects of wool incorporation, despite heterogeneity in materials, test standards, and reporting practices.

The distributions indicate that wool incorporation can produce measurable mechanical gains, but outcomes are strongly property- and system-dependent. Failure-stress metrics (tensile and flexural stress at failure) tend to show positive central tendencies in many studies, yet negative Δ% values also occur, underscoring that performance improvements are not universal. Modulus-based metrics exhibit the widest dispersion and the most extreme positive Δ% values, which is consistent with stiffness being highly sensitive to fiber dispersion, porosity, and interface quality, and can also reflect amplification when the matched-control baseline is low. Impact/energy-absorption outcomes display the largest apparent improvements but also substantial variability and typically fewer directly comparable datapoints, so these results should be interpreted as promising but less robust than the broader failure-stress evidence. Because a small number of extreme values can dominate scaling, the x-axis is truncated for readability and whiskers extending beyond the plotted range are clipped, without altering the underlying summary statistics.

To complement this distribution-level view,

Table 3. Top three wool-containing formulations by relative performance gain (Δ%) versus their corresponding control, grouped by mechanical property.

Bucket	Formulation Description	Δ (%)	Ref.
Compressive stress	Clay–cork + 30 wt% quick-lime + 2 wt% sheep-wool fibers (Process P3)	71	[37]
	Clay–cork + 30 wt% quick-lime + 1 wt% sheep-wool fibers (Process P3)	65	[37]
	Clay–cork + 10 wt% quick-lime + 2 wt% sheep-wool fibers (Process P3)	56	[37]
Flexural stress	Wool/polypropylene (PP) = 85/15 (w/w) (Process P2)	677	[38]
	Wool/PP = 80/20 (w/w); short sheep-wool fibers + PP fibers (matrix) (Process P2)	625	[38]
	Wool/PP = 90/10 (w/w) (Process P2)	438	[38]
Flexural modulus	Wool/PP = 80/20 (w/w); short sheep-wool fibers + PP fibers (matrix) (Process P2)	12,177	[38]
	Wool/PP = 85/15 (w/w) (Process P2)	8304	[38]
	Wool/PP = 90/10 (w/w) (Process P2)	8262	[38]
Tensile stress	Natural rubber 100 phr + ZnO 5, stearic acid 2.5, Wingstay L 1, CBS 1.5, sulphur 2.5 + wool 100 phr (coarse wool, chopped ~1.5 cm (Process P5)	234	[23]
	Epoxy + 4 fabric plies; Jute/Wool/Wool/Jute; fibers (jute+wool) (Process P6)	122	[39]
	10 wt% wool fiber (NaOH-treated, 30 min) + 30 wt% glass fiber + 58 wt% polyester + 2 wt% Al2O3 (Process P1)	107	[10]
Tensile modulus	Wool/PP = 80/20 (w/w); short sheep-wool fibers + PP fibers (matrix) (Process P2)	394	[38]
	Wool/PP = 85/15 (w/w); short sheep-wool fibers + PP fibers (matrix) (Process P2)	267	[38]
	Wool/PP = 90/10 (w/w); short sheep-wool fibers + PP fibers (matrix) (Process P2)	252	[38]
Impact (Izod)	10 wt% wool fiber (NaOH-treated, 30 min) + 30 wt% glass fiber + 58 wt% polyester + 2 wt% Al2O3 (Process P1)	464	[10]
	20 wt% wool + 20 wt% glass + 58 wt% polyester + 2 wt% Al2O3 (Process P1)	426	[10]
	40 wt% wool + 58 wt% polyester + 2 wt% Al2O3 (glass 0%) (Process P1)	128	[10]

P1: Hand lay-up followed by compression molding; cured for ~6 h at room temperature. P2: Carded into fiber “webs” and recombined; compression molding at 2000 psi for 8 min; no compatibilizer or flame retardant. P3: Oven-dried for 1 day to constant mass prior to testing. P4: UV-A/UV-B exposure (artificial weathering) for 12 days. P5: Mixed on a two-roll mill; cured at 150 °C; pre-conditioned at 25 °C/65% RH before testing. P6: Alkali treatment (1 wt% NaOH, 1 h), neutralized with 1% HCl, oven-dried ~10 h; hand lay-up laminate; pressed/cured under 50 kg for 24 h.

lists the top three best-case wool-containing formulations (highest Δ%) reported for each mechanical bucket, together with the associated study reference and processing route.

Table 3 illustrates that the most pronounced Δ% gains are typically tied to specific material–process combinations rather than wool addition alone. Compressive-stress best cases are comparatively moderate (+56 to +71%) and all stem from the same clay–cork system stabilized with quick-lime (Process P3), indicating that stabilization and moisture/curing control dominate these gains as much as (or more than) fiber reinforcement. In contrast, the most extreme flexural improvements cluster within a single thermoplastic route based on carded wool/PP fiber webs consolidated by compression molding (Process P2), yielding very large best-case increases in flexural stress (+438 to +677%) and especially flexural modulus (+8262 to +12,177%). These modulus values should be interpreted with caution because they are strongly amplified by the matched-control baseline used in the source study: wool/PP panels exhibit flexural moduli on the order of hundreds of MPa (≈393–577 MPa), whereas the commercial gypsum board comparator reports a modulus of only a few MPa (≈4.7 MPa), so a moderate absolute stiffness difference translates into four-digit percentage gains. Tensile-stress best cases are more distributed across matrix families, with the highest reported gain in a high-loading natural-rubber green composite (Process P5), followed by a hybrid jute/wool epoxy laminate (Process P6) and a polyester/glass–wool hybrid system. Impact best cases are dominated by polyester/glass–wool formulations (Process P1), underscoring that apparent toughness gains may be driven by hybrid reinforcement (and the wider additive/processing package) as much as by wool itself.

Table 3 reinforces the central finding of this subsection: substantial mechanical gains are achievable, but their magnitude (and sometimes direction) remains highly conditional on reinforcement architecture (web/laminate vs. short fibers), processing/void control and, critically, the choice of matched baseline; therefore, the listed outcomes should be framed as best-case demonstrations rather than representative expectations across the corpus.

3.5. Functional Performance

Under functional testing, the literature shows a broad but clearly imbalanced coverage. Figure 8 indicates that most functional datasets come from polymer-based systems, which account for the bulk of reported measurements across thermal transport and fire-related metrics, while acoustic performance is documented in fewer studies and is therefore less represented across the corpus. Mineral-binder formulations contribute a smaller, more selective set of functional outcomes, and the remaining matrix families (biopolymers/proteins, elastomers, and “other”) appear only sporadically across functional domains. Taken together, this distribution cautions against sweeping conclusions and reinforces the need to rely on matched-control comparisons when interpreting functional effects.

To move from coverage to outcome, Figure 9 summarizes functional performance as the relative change (Δ%) of wool-containing formulations with respect to their matched controls across the main functional buckets. The box-and-whisker format reports the median, interquartile range, and overall spread, allowing typical responses and variability to be compared on a common basis across studies.

The distributions reveal strongly domain-dependent responses to wool incorporation. Water-related metrics show the widest spread and the largest positive Δ% values, indicating that increases in water absorption are frequently reported and can be substantial, although negative values also occur, reflecting occasional reductions under specific formulations or treatments. Thermal conductivity changes cluster closer to zero with comparatively tighter dispersion, suggesting that reported effects are generally modest and sensitive to competing microstructural mechanisms (e.g., porosity and moisture content versus fiber–matrix continuity). Acoustic performance (sound absorption) tends to shift positively where reported, but the limited number of datapoints reduces the robustness of this trend. Fire-related outcomes are mixed: time to ignition often shows both gains and losses, while heat-release indicators (PHRR and THR) exhibit broad variability, consistent with strong dependence on matrix chemistry, additives/flame-retardant strategies, and specimen structure. Taken together, Figure 9 indicates that functional benefits are achievable but are less uniform than mechanical trends, with particularly high sensitivity to moisture uptake/porosity control and to the specific fire-testing context and formulation choices.

To complement the distribution-level synthesis,

Table 4. Top three wool-containing formulations by relative performance gain (Δ%) versus their corresponding control, grouped by functional property.

Bucket	Formulation Description	Δ (%)	Ref.
Thermal conductivity	Wool fiber length 1 mm; 46 wt% (of cement); washed	−92.00	[34]
	Wool fiber length 6 mm; 13 wt% (of cement); washed	−87.30	[34]
	Clay–cork + 30 wt% quick-lime + 2 wt% sheep-wool fibers (Process PR1)	−34.87	[37]
Sound absorption (α)	2.5% modified wool fiber (fiber volume fraction (VF)); 35% salt, 24 h; fibers 60–70 mm; concrete matrix (Process PR2)	200.00	[27]
	1.0% modified wool fiber (Vf); 35% salt, 24 h; fibers 60–70 mm; concrete matrix (Process PR2)	200.00	[27]
	1.0% wool fiber (Vf); fibers 60–70 mm; unmodified; concrete matrix (Process PR2)	164.00	[27]
Time to ignition (TTI)	92% PLA + 3 wt% P-TAB + 5 wt% wool fiber	22.50	[40]
	87% PLA + 3 wt% P-TAB + 10 wt% wool fiber	20.00	[40]
	PP + 30 wt% kenaf fiber + 20 wt% ammonium polyphosphate + 3 wt% wool fiber	16.67	[41]
Peak heat-release rate (PHRR)	Wheat gluten (WG) + 20 wt% glycerol + 30 wt% wool; 140 °C (Process PR3)	−58.88	[26]
	WG + 20 wt% glycerol + 30 wt% wool; 160 °C (Process PR3)	−56.84	[26]
	WG + 30 wt% glycerol + 30 wt% wool; 140 °C (Process PR3)	−55.87	[26]
Total heat release (THR)	77% PLA + 3 wt% phenylphosphonic 3(2-aminobenzothiazole) (P-TAB) + 20 wt% wool fiber	−42.86	[40]
	87% PLA + 3 wt% P-TAB + 10 wt% wool fiber	−30.61	[40]
	92% PLA + 3 wt% P-TAB + 5 wt% wool fiber	−18.37	[40]
Water absorption	Wool/PP = 80/20 (w/w); short wool fibers + PP fibers (matrix) (Process PR4)	−75.00	[38]
	Wool/PP = 90/10 (w/w); short wool fibers + PP fibers (matrix) (Process PR4)	−59.52	[38]
	Coir/Sheep wool/Gypsum 10/20/70 (w/w%)	−55.56	[33]

PR1: Air-cured 28 d at 23 ± 2 °C, RH 60 ± 10%; oven-dried 1 d to constant mass before tests. PR2: Water curing until test day. PR3: Compression-molded panel. PR4: Carded into fiber “webs” and recombined; compression molding at 2000 psi for 8 min; no compatibilizer or flame retardant.

highlights the top-performing wool-containing formulations within each functional bucket, reporting the three most favorable Δ% outcomes relative to their corresponding controls, together with the study reference and a simplified process classification. Because “improvement” is direction-dependent for functional metrics, reductions (negative Δ%) are reported as the best cases for properties where lower values are beneficial (e.g., thermal conductivity and heat-release metrics), whereas increases (positive Δ%) represent best cases for domains such as sound absorption.

Table 4 indicates that functional best-case outcomes are highly dependent on the targeted property and on tightly coupled material–process–additive configurations, rather than being an intrinsic, transferable “wool effect”. The strongest reductions in thermal conductivity are concentrated in mineral-binder systems (notably cementitious mixes with washed short fibers), consistent with insulation gains being governed largely by bulk density/porosity shifts and disruption of heat-transfer pathways. The largest improvements in sound absorption (α) are likewise restricted to a small subset of concrete mixes using very long fibers, suggesting that pore structure and tortuosity—strongly influenced by fiber morphology and mix design—dominate acoustic performance. Fire-related best cases further show formulation control: the highest increases in time to ignition (TTI) occur in FR-containing PLA and in an additive-modified PP hybrid, while the strongest reductions in PHRR/THR appear in specific biopolymer/FR routes, implying that ignition delay and heat-release suppression can be optimized through different levers and should not be assumed to co-vary. Finally, the most favorable reductions in the water absorption cluster in selected thermoplastic/gypsum-based systems where fiber encapsulation and void control are likely decisive.

Table 4 supports the broader conclusion that functional benefits are demonstrable, but they should be interpreted as best-case demonstrations within matched-control comparisons, not as representative expectations across the corpus.

3.6. Applications and Maturity

To translate the mechanical and functional evidence into practical targets, this subsection summarizes the application contexts explicitly stated in the corpus and assesses how close the reported evidence is to use-oriented validation. Because application claims are not uniform across studies and prototypes are not always tested against directly comparable references, maturity is interpreted here in terms of reporting quality indicators (availability of comparable controls and explicit test standards) rather than formal TRL.

Table 5. Application fit and maturity snapshot derived from the Table S1_MasterTable.

Application/Product type	Typical Matrices & Processes	Evidence Focus (Most Reported Outcomes)	Key Trade-Offs/Gaps	Maturity Snapshot
Automotive components (interiors & crashworthiness)	Thermoset; Thermoplastic; Biopolymer; Hand lay-up/lamination; Compression molding/hot press; Melt mixing/thermoforming	Tensile stress/modulus; flexural stress/modulus; impact; SEM (fractography); crashworthiness metrics (quasi-static axial crushing, peak load, SEA) where applicable; fire performance in FR bioplastics (UL-94, cone calorimetry) when reported	High sensitivity to formulation and fiber/matrix interface; comparability limited by inconsistent reporting of fiber treatment, conditioning and replicate/uncertainty; crashworthiness outcomes strongly geometry- and trigger-dependent	n = 7; comparable controls = 57%; standards = 86%
Construction/building products	Mineral binder; Thermoset; Thermoplastic; Cement/gypsum mixing–casting; Hand lay-up/lamination; Melt mixing/thermoforming	Flexural stress/strength; compressive stress/strength; water absorption/moisture uptake; workability/density/porosity; thermal conductivity where reported	Moisture uptake/durability often critical; performance often dominated by porosity and fiber dispersion; incomplete reporting of mix design and curing/conditioning can limit cross-study comparability	n = 14; comparable controls = 79%; standards = 100%
Insulation/acoustic materials	Mineral binder; Thermoset; Other/Biopolymer; Cement/gypsum mixing–casting; Hand lay-up/lamination; Other/unclear	Thermal conductivity; sound absorption coefficient (α/NRC); density/porosity; water absorption/moisture content; compressive/flexural properties for handling/installation; thermal stability (TGA) when reported	Trade-offs between insulation/acoustic gains and mechanical integrity; moisture sensitivity frequently relevant; controls and standardized acoustic/thermal test reporting are not uniform across studies	n = 10; comparable controls = 50%; standards = 100%
Other/mixed	Thermoset; Thermoplastic; Hand lay-up/lamination; Compression molding/hot press; Melt mixing/thermoforming	Tensile stress/modulus; flexural stress/modulus; thermal stability (TGA/DSC); water absorption; morphology (SEM) where reported	Application framing is often generic, limiting translation; high heterogeneity in formulations and baselines; incomplete reporting of conditioning/replicates can weaken interpretability	n = 12; comparable controls = 50%; standards = 92%
Textiles/nonwovens	Elastomer (natural rubber) compounding/vulcanization; textile reinforcement architectures	Tensile (ASTM D412); tear (ASTM D624); hardness (ASTM D2240); density (ASTM D792)	Single-study evidence; outcomes strongly influenced by fiber dispersion and vulcanization conditions; limited comparability beyond the tested compound and protocol	n = 1; comparable controls = 100%; standards = 100%

Application clusters were coded from the Table_S1_MasterTable Application_domain field using keyword-based grouping. “Comparable controls” denotes studies reporting a directly comparable baseline (as coded in the MasterTable), and “Standards” denotes explicit reporting of test standards. “Evidence focus” lists the most frequently reported outcomes within each application cluster.

consolidates this information by grouping the stated application domains into higher-level product types and summarizing, for each cluster, the dominant matrix–process combinations, the outcomes most often reported, and a simple maturity snapshot based on study counts and the proportion of studies reporting comparable controls and relevant standards.

Two patterns emerge. First, construction and building products form the largest stated application cluster (n = 14), spanning cementitious, gypsum and earthen systems alongside a smaller set of polymer laminate and polymer-processing routes. This cluster also shows comparatively stronger reporting maturity: standards are consistently specified and most studies include directly comparable controls (79%), supporting controlled-comparison interpretation for flexural/compressive stress-at-failure and for thermal–moisture-related behavior.

Second, non-construction targets are more fragmented and typically show weaker control comparability. Automotive components (interiors and crashworthiness; n = 7) are mainly polymer-based and emphasize stress/stiffness and impact-related outcomes, but standards reporting is less consistent than in construction (86%), and only 57% of studies include directly comparable controls. Insulation/acoustic products (n = 10) focus on thermal conductivity, sound absorption and density/porosity, yet only half the studies provide comparable controls, so performance claims should be treated as preliminary unless controls and conditioning are clearly aligned. “Other/mixed” (n = 12) is a substantial but heterogeneous bucket with similar control limitations, reinforcing the need for clearer control definitions, consistent conditioning reporting, and durability/aging validation before broad application-level generalizations are made.

4. Discussion

The evidence map (2015–2025; n = 44) confirms that “wool-reinforced composites” is not a single materials class but a two-stream design space dominated by (i) polymer matrices (especially thermosets and, to a lesser extent, thermoplastics), and (ii) mineral binders for construction (cementitious, gypsum, and earthen systems). Across both streams, the dominant wool architecture is short fibers, while woven/nonwoven forms appear mainly where laminate-like processing or insulation-oriented products are targeted. This structural heterogeneity—together with inconsistent reporting of reinforcement fraction (wt% vs. vol%), moisture conditioning, and porosity/void metrics—explains why meta-analysis is still hard to justify and why matched-control Δ% summaries are a practical synthesis strategy for the present corpus.

4.1. Mechanical Response Is Real, but Conditional on Interface and Process “Defects”

The Δ% distributions indicate that wool can deliver substantial improvements in impact/energy absorption and, in favorable cases, increases in tensile/flexural stress at failure (often reported as “strength”). However, negative Δ% values remain common in the same buckets, implying that gains are not intrinsic to wool addition but emerge when dispersion, wetting/impregnation, and void control are achieved.

This aligns closely with broader natural-fiber composite reviews: mechanical scatter is often governed less by fiber nominal properties and more by fiber–matrix adhesion, fiber damage/attrition during processing, and moisture-driven interfacial degradation [2,42,43]. In thermosets, hand lay-up/infusion routes amplify sensitivity to fiber cleanliness and entrapped air; in thermoplastics, melt compounding adds fiber shortening, thermal history, and moisture volatilization as additional defect sources. The extreme best-case modulus gains observed in limited thermoplastic systems should therefore be interpreted as system-specific demonstrations (potentially inflated by low baseline controls and morphology changes) rather than representative expectations—an interpretation consistent with general NFC literature on stiffness being disproportionately affected by voids, fiber orientation, and baseline selection [2,42].

4.2. Moisture and Durability Are the “Translation Bottleneck”

Across the functional buckets, water-related metrics show the widest dispersion and often large increases, indicating that moisture sensitivity is a recurring trade-off. This is consistent with a well-established NFC failure pathway: fiber hygroscopicity and swelling lead to microcracking and interfacial debonding, as well as time-dependent loss of properties under water/hygrothermal cycling and weathering [43,44]. For wool specifically, keratin’s chemistry can be advantageous for some functional responses, but moisture management remains central: processing steps that reduce free water (thorough drying), improve encapsulation, or tailor the interface (e.g., silanes/compatibilizers, controlled alkalization where appropriate) are likely prerequisites for reliable structural claims. The implication for future wool-composite studies is that “water absorption” should not be treated as a secondary characterization metric: it should be paired with retained mechanical performance after conditioning, at least under a minimal durability protocol (water immersion/hygrothermal + post-exposure tensile/flexural stress). This is the same direction recommended by recent reviews focused on moisture effects and durability enhancement in natural-fiber composites [45].

In this context, wool’s biodegradability is best interpreted as a moisture-mediated risk: keratin can be degraded by keratinolytic microorganisms when water and access pathways are available, whereas dense matrices with good encapsulation can limit bio-accessibility [46]. Accordingly, durability claims—especially for cementitious systems—should consider that sustained wet curing and highly alkaline pore solutions have been reported to accelerate wool-fiber degradation [47].

4.3. Functional Performance Depends on Bulk Architecture More than Fiber Identity

Thermal conductivity shifts are often modest and clustered near zero in polymer systems, while the strongest reported reductions appear in mineral-binder formulations where wool contributes to lower density and higher porosity architectures. This pattern aligns with construction-material reviews showing that thermal and acoustic performance in fiber-reinforced mortars/concretes is frequently governed by pore structure (and its coupling to moisture), sometimes more than by fiber type alone [48,49]. For acoustics, the positive shifts in sound absorption (where reported) are plausible because fibrous/porous morphologies promote viscous and thermal dissipation; this agrees with the broader literature on acoustic materials to do with fibrous absorbers and natural-fiber acoustic composites [50,51,52]. However, because acoustic outcomes are presently supported by fewer directly comparable datasets, claims should be framed as promising but under-validated, ideally requiring standardized specimen geometry, airflow resistivity or porosity descriptors, and frequency-resolved reporting.

4.4. Fire Behavior: Wool Can Help, but “System Fire Safety” Is Formulation-Driven

Fire-related outcomes are mixed in the Δ% synthesis, which is expected: fire performance in composites is strongly shaped by matrix chemistry, additive packages, char formation, dripping behavior, and specimen thickness/geometry. Wool’s keratin content is often cited as beneficial (e.g., char and nitrogen/sulfur-containing structures), but the corpus indicates that wool alone is not a universal fire solution; the best cases typically emerge in coupled strategies (e.g., wool + FR additives in PLA/protein systems). This aligns with reviews on flame retardancy in natural-fiber polymer composites, which emphasize that improvements in TTI/PHRR/THR depend on the whole formulation and on balancing fire performance with mechanical integrity and aging resistance [53,54,55]. A practical implication for future wool-based studies is to report fire tests with sufficient context (heat flux, thickness, orientation, conditioning), and—when fire is an application driver—to include mechanical retention after aging to avoid “fire-safe but moisture-fragile” designs.

Compared with cellulose/lignocellulosic fibers, wool’s keratin chemistry (N/S-containing structures) tends to favor char-forming pathways that can be beneficial in fire-oriented formulations, although the net response remains system- and additive-dependent [26,53,55]. However, wool typically offers a narrower thermal-processing window than many plant fibers; in practice, natural-fiber thermoplastic processing is often recommended to remain below ~200 °C to limit fiber degradation, which can restrict wool use in high-melting polymers or long-residence-time compounding [40,56]. Therefore, wool-based biocomposites are best positioned as complementary to cellulose-based systems—trading some high-temperature processability for potentially favorable fire-related behavior—while still requiring moisture/interface control for durable performance [43,45].

4.5. Application Fit: Construction Dominates; Automotive Is Promising but Needs Durability + Fire Coherence

The application snapshot indicates that construction/building products form the largest single cluster (n = 14), followed by “other/mixed” (n = 12) and insulation/acoustic products (n = 10), while automotive components remain smaller (n = 7). However, Table 5 also shows that insulation/acoustic studies frequently report standards but often lack directly comparable controls, whereas automotive studies more often include controls but show less consistent standards reporting, so “maturity” differs by indicator rather than by application volume alone.

This is coherent with the external review literature: animal fibers (including wool) have established momentum in cementitious/gypsum/soil matrices because low-to-moderate mechanical demands can be traded for insulation/acoustic gains and circularity narratives [4,49]. For automotive/interior components, broader reviews on natural-fiber composites consistently highlight lightweighting potential, but also flag durability (humidity/temperature cycling) and flammability compliance as key adoption barriers—exactly the two axes where wool-composite results currently show the highest variability [57].

4.6. Reporting Priorities to Unlock Comparability (and Make TRiC “High” Achievable)

Given the observed dispersion and the constraints on cross-study synthesis, the most leverage comes from improving comparability by design. Minimum-reporting items that would immediately raise reproducibility include: (i) fiber state (raw/scoured, diameter/length distribution, moisture content at processing), (ii) reinforcement fraction reported in both wt% and vol% (or sufficient densities to convert), (iii) processing parameters that control voids (degas, pressure/temperature profiles, curing/compaction), (iv) conditioning protocols before testing, and (v) explicit matched controls made from the same batch and tested under identical conditions. These priorities mirror recommendations repeatedly made in natural-fiber composite reviews (interface/moisture/fire) and would directly reduce the uncertainty bands observed in the Δ% boxplots [43,53].

The corpus supports a balanced conclusion: wool is a credible reinforcement/functional modifier with real upside—especially for toughness, acoustic response, and low-density construction products—but the dominant determinants of success are process-enabled microstructure (dispersion, wetting, porosity) and service-environment robustness (moisture aging and fire context). Future work that treats wool fibers as an engineered phase (with controlled interface and conditioning) rather than as a generic “waste filler” is the most direct route to turning best-case demonstrations into reliable, application-grade materials.

4.7. Evidence-Based Design and Reporting Guidelines

To help translate a heterogeneous evidence base into practical design choices,

Table 6. Evidence-based guidelines derived from Table S1 (W001–W044).

Guideline (Actionable Statement)	Applies Mainly to	Evidence Signal (Median Δ%, IQR; n)	Practical Caveats/ Boundary Conditions
G1. If thermal insulation is the target in mineral binders, wool is a robust lever—but do not assume compressive gains	Mineral binders (mortars, gypsum, cementitious)	Thermal conductivity: −26.1% (IQR −34.9 to −13.4), n = 9; Compressive stress: −11.7% (IQR −25.3 to +8.0), n = 29	Compressive-stress penalties can be coupled to density and porosity changes; report density plus curing and conditioning to enable fair comparisons
G2. In thermosets, wool can deliver large stress improvements, but moisture-related performance must be managed explicitly	Thermosets (epoxy, polyester; laminates)	Tensile stress: +54.6% (IQR +29.5 to +106.7), n = 5; Flexural stress: +109.9% (IQR +81.6 to +133.7), n = 5; Water absorption: +224.2% (IQR +97.9 to +298.7), n = 18	Report conditioning protocol; consider barrier coatings or interface strategies for humid exposure
G3. For thermoplastics, prioritize interface control and moisture management; expect modest stiffness changes unless reinforcement is well-coupled	Thermoplastics	Flexural modulus: +8.0% (IQR −4.5 to +9.0), n = 9. Tensile stress datapoints were flagged as non-comparable controls in the dataset	Do not generalize tensile outcomes without matched controls; report compatibilizer use, fiber drying, and processing severity
G4. For fire behavior, wool often reduces PHRR, but overall improvement requires multi-metric verification (PHRR, TTI, THR)	Thermoplastics and biopolymers with flammability testing	PHRR: Thermoplastic −30.5% (IQR −35.6 to −22.6), n = 8; Biopolymer −55.5% (IQR −56.1 to −54.8), n = 8 Biopolymer TTI: −6.8% (IQR −9.3 to −3.1), n = 8; Biopolymer THR: +15.2% (IQR +13.7 to +17.8), n = 8	Avoid single-metric claims; report specimen thickness, heat flux, and standard; check trade-offs between ignition time and total heat release
G5. Choose wool architecture to match the load path: textiles and mats can raise stress, but chopped fibers are dispersion-limited and outcome-volatile	Polymer composites (architecture-sensitive systems)	Across studies, stress gains are more consistently reported when architecture enables load transfer (woven, laminate, nonwoven); short-fiber systems show higher scatter and processing sensitivity	Report fiber length distribution, areal weight (textiles), orientation, and processing-induced damage to avoid non-combinable datasets
G6. Reporting guideline (to make results usable in future synthesis): always publish the minimum comparability set	All matrix families	Not a performance metric; directly affects interpretability and inclusion in quantitative synthesis	Minimum set: matched control definition; wool content (wt% and/or vol%); wool form and size; interface treatment; process route; test standard; conditioning; specimen geometry; n and dispersion; plus density and porosity where relevant

distils patterns observed in the extracted dataset (Table S1; W001–W044) into a small set of evidence-based guidelines. Each guideline is anchored to control-matched comparisons using the relative change versus the corresponding control formulation (Δ%). Wherever possible, the evidence signal is summarized using the median and interquartile range (IQR) across datapoints where the control was judged comparable, alongside the number of datapoints (n). These guidelines are intended as decision heuristics rather than universal rules, and should be interpreted in light of formulation, processing, and testing variability across studies.

Table 6 highlights that the most defensible design choices are matrix- and property-dependent. In mineral binders, wool inclusion is most consistently associated with improved thermal insulation, while compressive stress typically decreases and shows wide dispersion, indicating strong sensitivity to density/porosity and curing. In thermosets, the evidence supports substantial gains in tensile and flexural stress, but these benefits commonly coexist with large increases in water uptake, making conditioning and moisture management first-order design constraints. For thermoplastics, the dataset shows modest median-stiffness changes and underlines the need for transparent, matched controls (particularly for tensile outcomes) before generalization. For fire behavior, reductions in PHRR are frequent, but multi-metric interpretation is essential, since some systems show trade-offs between ignition time and total heat release; therefore, claims should be supported by the full set of relevant fire metrics under clearly reported test conditions.

5. Conclusions

This systematic review synthesized 44 journal studies (2015–2025) on raw or minimally processed sheep-wool reinforcement in polymer matrices (33/44) and mineral-binder construction materials (11/44). Because formulations, processing routes, and test conditions are highly heterogeneous—and only a subset of studies report directly comparable matched controls—conclusions are most defensible as conditional patterns rather than pooled effect sizes. Across material families, wool can be a viable reinforcement and functional modifier, but reproducible performance depends primarily on process-enabled microstructure control (dispersion/impregnation and void/porosity management) and service-environment robustness.

• Evidence landscape: The field forms a two-stream design space (polymer vs. mineral-binder systems), with short fibers being the dominant architecture and textiles/nonwovens used mainly in laminate and insulation contexts.
• Mechanical response is real but conditional: The most repeatable upside is improved toughness/impact and energy absorption, while tensile/flexural responses range from beneficial to detrimental depending on interface quality, fiber damage, wetting/impregnation, and porosity. Negative shifts are consistent with microstructural “defects” rather than with wool fiber identity alone.
• Moisture durability is the key translation bottleneck: Water-related outcomes show the widest dispersion. Application-relevant claims require mechanical retention after conditioning (immersion/hygrothermal exposure), not moisture uptake metrics alone.
• Functional properties are system- and architecture-dependent: Thermal and acoustic outcomes are most defensible when interpreted at the level of bulk architecture and density/porosity, while fire behavior should be treated as formulation-driven and supported by multi-metric evidence under a fully reported test context.
• Highest-leverage next step—comparability by design: To make results cumulative and enable stronger synthesis, future studies should adopt a minimum reporting and experimental-design set: (i) matched controls tested identically; (ii) reinforcement fraction reported in wt% and vol% (or densities enabling conversion); (iii) explicit conditioning protocols; (iv) void/porosity descriptors; (v) test standards and conditions; and (vi) replication/dispersion reporting.

In practice, the evidence supports the following decision rules: mineral binders are the most defensible route when low-density/insulating construction performance is the priority (without assuming compressive stress gains); thermosets can deliver large tensile/flexural improvements when interface and voids are controlled, with conditioning-based retention as a first-order constraint; thermoplastics typically show modest stiffness shifts unless coupling/processing is engineered; and fire-related claims should be based on multi-metric evidence (e.g., PHRR with TTI and THR) under a fully reported context. Overall, treating wool as an engineered phase (controlled interface, controlled architecture, and conditioning-aware testing) is the most direct route to translating best-case demonstrations into application-grade materials.