Material Composition Testing Related to Measurement Instrument Enclosure Design and Safety

Abstract

The polymeric insulating materials widely used in measuring instrument enclosures must meet specific flammability requirements. In this study, we systematically assessed the impact of minor compositional changes, such as pigments and fillers, on horizontal burning (HB) classification according to the EN 60695-11-10 standard. We tested 64 polymer combinations at thicknesses of 1.5 mm and 3.0 mm, classifying samples into HB, HB40 or HB75 categories. The results demonstrated that additives significantly influenced the HB classifications more than thickness. Specifically, we classified 31 samples as HB, 16 as HB40, 15 as HB75, while two were unclassifiable. Several material groups consistently achieved specific HB classifications regardless of minor additive variations. These findings offer manufacturers clear guidance for selecting polymer-additive systems, facilitating informed decisions and enabling a streamlined “worst-case” testing strategy. Ultimately, this approach enhances manufacturers’ ability to efficiently achieve product safety compliance, reducing certification costs without compromising safety.

1. Introduction

Polymeric insulating materials are widely used as enclosures in electrical and measuring instruments due to their lightweight, moldability, and electrical insulation properties. However, the fire safety of these enclosures is a critical concern because an enclosure that ignites can lead to device failure and even propagate fire to its surroundings. Ensuring the flame resistance of enclosure materials is not only a matter of product reliability but also a regulatory requirement. For example, the European Measuring Instruments Directive 2004/22/EC (MID) mandates that measuring devices “constitute no unreasonable risk to personal safety or property,” which includes meeting basic fire safety criteria [1,2,3]. Manufacturers must therefore design enclosures with materials that resist ignition and self-extinguish, in order to comply with such directives and obtain the necessary safety certifications for global markets. This has placed increased importance on selecting or formulating polymer materials that can meet flammability standards while retaining the mechanical and functional requirements of the instrument enclosure.

To evaluate and classify the flammability of plastics used in enclosures, standardized small-flame tests are employed worldwide. The most common are the horizontal and vertical burning tests defined in Underwriters Laboratories UL 94 and the equivalent IEC 60695-11-10 international standard [4]. In particular, European norm EN 60695-11-10 (IEC 60695-11-10) specifies a 50 W Bunsen burner method for comparing the burning behaviour of plastic specimens in horizontal (HB) and vertical (V) orientations [3,4]. In the horizontal burning test (known as UL 94 horizontal burning (HB) or method A), a bar-shaped sample is held horizontally and exposed to a calibrated 50 W flame for a fixed interval. The rate and extent of burning are then measured to categorize the material’s flammability. Materials that stop burning before a specified distance or burn slowly are classified in increasing order of flammability as HB40 or HB75, corresponding to the maximum allowed burning rates of 40 mm/min and 75 mm/min, respectively. These HB ratings indicate a material’s ability to self-extinguish or burn at a controlled rate when ignited horizontally. Standards IEC 60695-11-4 and -11-10 define the test apparatus and procedures in detail to ensure reproducibility, including the burner specifications, flame height (20 mm), gas purity, and the conditioning of samples [5]. Using these methods, manufacturers and researchers can quantitatively compare the flammability of different polymer materials and verify their compliance with safety norms.

A substantial body of research has examined how the composition of polymeric materials influences their flammability, as measured by such standardized tests. It is well known that base polymers vary widely in inherent flammability—for instance, unfilled polycarbonate or epoxy tend to char and self-extinguish, whereas materials like polyethylene or Acrylonitrile Butadiene Styrene (ABS) can burn readily [6]. To meet safety requirements, flame-retardant additives are often incorporated into polymers to reduce their ignitability and burning rate. Halogenated flame retardants, organophosphorus compounds, and metal hydroxide fillers (e.g., alumina trihydrate or magnesium hydroxide) are commonly used to impart self-extinguishing behaviour [7]. These additives act through various mechanisms (gas-phase radical quenching, char formation, endothermic cooling, etc.) and can dramatically improve a material’s rating and HB in many cases. At the same time, additives and fillers can also affect the thermal stability and mechanical properties of the polymer, so their selection involves balancing flame retardancy with other performance factors. Recent studies have explored more sustainable or halogen-free flame retardants due to environmental and health concerns. In contrast to these well-studied flame-retardant systems, there is comparatively less literature on the flammability of minor formulation ingredients such as color pigments or fillers not primarily added for fire safety. These additives, often present for aesthetic or processing reasons, can nonetheless influence burning behaviour by changing the polymer’s thermal conductivity, degradation pathway, or char formation characteristics [8].

The influence of pigments and inert fillers on polymer flammability has yielded some intriguing and sometimes divergent findings. Barbot’ko et al. [8] investigated methylsiloxane rubber compounds with various inorganic pigments and found that most mineral-based colorants (e.g., iron oxide, titanium oxide, etc.) did not significantly change the material’s horizontal burning rate compared to the unpigmented base polymer. Notably, however, the addition of carbon black pigment (nanoscale carbon) was observed to improve the residual smoldering behaviour and slightly reduce the burning time, likely by promoting char formation. Some research highlights that even additives introduced for non-safety reasons (color, UV stability, fillers from recycled content, etc.) may alter a material’s flammability—sometimes for the better, sometimes for the worse. Indeed, the effect of inert fillers on fire performance can be complex: adding low thermal conductivity fillers can slow heat propagation and promote char (a positive effect), whereas other fillers might create additional combustible volatiles or disrupt char integrity (a negative effect) [9].

Despite the extensive research on flame retardant formulations, there remain several challenges in the fire testing of polymer blends and composites, especially in HB rating, which is a research gap. Another challenge lies in the consistency of test outcomes when materials are modified. Even minor changes in the polymer grade, additive type, or colorant could potentially change the HB rating, sometimes unexpectedly.

Given the need for the safe yet efficient design of instrument enclosures, one practical question for manufacturers is whether material formulations can be optimized to meet flammability criteria without requiring a full battery of tests for every variant. If a family of materials—for example, a base polymer with different pigment packages—can be shown to consistently achieve the same flame classification, it would greatly streamline product development and certification. There is growing interest in strategies to reduce repetitive flammability testing while ensuring safety. On one front, researchers are exploring predictive modelling and simulation. Another approach is to empirically establish “worst-case” compositions: this includes, for instance, determining if the darkest pigment or highest filler loading still passes the HB test, which would imply that any lighter pigmentation or lower loading would also pass. If such worst-case testing can be validated, standards could allow the grouping of material variants, reducing the need to test each color of an enclosure separately. In the context of measuring instrument enclosures, this is particularly relevant—manufacturers often offer devices in multiple colors or with minor formulation tweaks, and avoiding redundant flame tests for each variant would save time and costs while still meeting MID and other safety requirements. However, the regulatory acceptance of such an approach requires robust data to prove the equivalence of flammability performance.

Recent studies published in MDPI journals have significantly contributed to understanding material flammability and fire safety, covering diverse topics such as fire retardants in wooden structures [10], bio-based insulations for steel envelope systems [11], wood polymer composites [12], the real-scale analysis of fire dynamics [13], and fire-resistant enclosure panel systems [14]. Despite these contributions, research specifically addressing the horizontal burning behaviour (HB test method) of insulating materials for enclosures of measurement instruments, particularly regarding the impact of minor compositional changes such as pigments and fillers, remains largely unexplored. Thus, the current study aims to fill this research gap by systematically investigating whether combinations of common insulating materials with various additives consistently meet the HB, HB40, and HB75 flammability criteria, potentially streamlining safety certification processes for measurement instrument enclosures.

Motivation and Objective: The present study is situated in this context of balancing safety compliance with practical material selection. We focus on the horizontal 50 W flame test (EN 60695-11-10, method A) as a baseline flammability assessment for insulating enclosure materials. This was in order to systematically investigate how different polymer materials and their additive combinations influence the HB classification, and whether multiple formulations can be deemed equivalent in fire behaviour. Unlike most previous studies that concentrate on achieving the highest flame rating (V-0) using dedicated flame retardants, our work examines more subtle formulation changes (different base polymers, color pigments, and fillers) and their effect on the HB, HB40, or HB75 category achieved. We tested 64 material combinations, encompassing several common thermoplastic and thermoset polymers used in instrument casings, along with various pigment and filler additions. By analyzing such a broad matrix of compositions, we aim to identify patterns or groupings of materials that consistently exhibit the same flammability classification. The novelty of this research lies in its scale and its emphasis on “equivalent performance” among different compositions—an area not extensively covered in the literature. The ultimate objective was to provide guidance on material formulation for manufacturers: if certain additives or pigments are proven not to degrade (or even improve) the flame resistance of a base material, these could be interchanged with minimal safety risk, potentially obviating the need for separate certification tests for each variant. In summary, this study seeks to place fire safety compliance in a broader materials engineering context, exploring how the intelligent selection of polymer/additive combinations can maintain required flammability standards (HB, HB40, HB75) for measuring instrument enclosures without the repetitive testing of every minor variation. The findings contribute to both the scientific understanding of polymer flammability and the practical effort to optimize material design for safety, thereby supporting innovation in instrument enclosure design under the stringent requirements of regulations and standards. Although horizontal burning tests are commonly used to assess material flammability, the existing literature rarely addresses how minor changes in composition—such as the addition of pigments or inert fillers—affect HB classification. This study fills that gap by systematically investigating whether such compositional variations impact the horizontal burning behaviour of materials used in measurement instrument enclosures.

2. Testing Method

Testing Procedure for 50 W Horizontal Flame Test (method A) provides valuable data for comparing the relative burning behaviour of various insulating materials, controlling manufacturing processes, or assessing changes in burning characteristics. The horizontal orientation of the test specimen is specifically suited to evaluating both the extent of burning and the flame propagation velocity (linear burning rate).

The equipment used for performing the test included:

• Laboratory fume hood/chamber: Volume of 0.5 m³, with dark walls and recorded light levels below 20 lx, equipped with an exhaust fan (switched off during the test) and a positive closing damper.
• Laboratory burner compliant with IEC 60695-11-4 standard.
• Specimen holder with clamps to securely position the test specimen.
• Timing device with a resolution of at least 0.5 s.
• Measuring scale graduated in millimeters.
• Wire gauze (20 mesh), compliant with standards.
• Conditioning chamber-maintaining conditions of 23 ± 2 °C and 50 ± 5% relative humidity (RH).

The bar-shaped test specimens measured 125 ± 5 mm in length and 13.0 ± 0.5 mm in width, with a maximum thickness not exceeding 13 mm. The edges of the specimens were smooth, with corner radii not exceeding 1.3 mm. Alternative thicknesses were permitted by mutual agreement between relevant parties. A maximum of six specimens per material combination were prepared for testing. Figure 1 illustrates the positioning of a test specimen on the test stand, showing the wire gauze beneath the specimen and the burner in its required position.

Before testing, specimens were conditioned for 48 h in a controlled environment at 23 ± 2 °C and 50 ± 5% RH. Figure 2 presents the conditioning setup for the test specimens.

Figure 3 demonstrates the actual test apparatus setup during testing. We fixed each bar-shaped specimen horizontally on the stand, with the transverse axis inclined at an angle of 45 ± 2°. The wire gauze was positioned horizontally 10 ± 1 mm below the specimen’s lower edge.

For method A (horizontal burning), testing was conducted in laboratory conditions with ambient temperatures ranging from 15 °C to 35 °C and a relative humidity ranging from 45% to 75%. Each specimen was marked with two perpendicular lines relative to its longitudinal axis, located 25 ± 1 mm and 100 ± 1 mm from the ignition end. We applied the flame from the burner on the edge of the sample (as presented in Figure 1) steadily for 30 ± 1 s or removed it immediately once the flame front reached the 25 mm mark. The timing device was reset when the flame front reached the 25 mm mark. If the specimen continued to burn after flame removal, we recorded the burning duration from the 25 mm mark until it passed the 100 mm mark. If the flame front exceeded 25 mm but did not reach 100 mm, both the elapsed time and damaged length were recorded. If any specimen from the initial set of three failed to meet the classification criteria, a second set of three specimens was tested. All specimens from the second set had to meet the required criteria. The linear burning rate (v, in mm/min) for specimens whose flame front passed the 100 mm mark was calculated using the following:

v = (60 × l)/t

(1)

where v = linear burning rate (mm/min), l = damaged length (mm) and t = time (s).

Materials Classification Criteria

We classified materials according to EN 60695-11-10 into HB, HB40, or HB75 categories, based on the following criteria:

• HB classification: (a) The material does not visibly burn after the removal of the ignition source. (b) If burning continues, the flame front does not pass the 100 mm mark. (c) If the flame front passes the 100 mm mark, the linear burning rate must not exceed 40 mm/min for thicknesses between 3.0 mm and 13.0 mm or 75 mm/min for thicknesses below 3.0 mm. (d) If the linear burning rate does not exceed 40 mm/min at a thickness of 3.0 mm ± 0.2 mm, the material is automatically accepted down to a minimum thickness of 1.5 mm.
• HB40 classification: (a) The material does not visibly burn after the removal of the ignition source. (b) If burning continues, the flame front must not pass the 100 mm mark. (c) If the flame front passes the 100 mm mark, the linear burning rate must not exceed 40 mm/min.
• HB75 classification: (a) The linear burning rate of the material must not exceed 75 mm/min if the flame front passes the 100 mm mark.

3. Results

3.1. Tested Materials

This section presents a detailed description of the tested materials and their combinations with additives.

Table 1. Identification of materials and additives.

	Ref. No.	Material (Provided by Metrel-SLO, Slovenia)
materials	81001531	ABS Terluran GP 22
	81001295	ABS 100-x01 natur
	81001482	PC Makrolon 6557
	81001443	ABS VAMBSAB 0023
	81002009	TPE Megol I A 65 SV/P
	81001810	TPU Elastollane S 90 A 15000
	81000936	PA6 Bergamid B 70 UF dark grey VN 5371 CF
	81000935	PA6 Bergamid B 70 UF blue VN 537o CF
	10223024	PA66 Ultramid A3K black
	10238024	PA66 Ultramind A3X2G5 black 23187
	81000264	PA66 Ultramind A3k natur
	81000863	PC makrolon 2805
	81001893	PC Lexan DMX 1435
	81000917	PP Hostancom M2 U01 natur
	81000918	PP Novolen 2300 K natur
	81000853	ABS Novodur P2H AT grey (RAL 7012)
	81000851	Plastic PC Makrolon 2607
additions	81001532	Pigment EUROCOLOR A92936 RAL 7021 4%
	81000577	HOSTATRON-SYSTEM P1935
	81001483	Pigment EUROCOLOR-GRIGIO A90917 RAL 7001 2%
	81001119	MASTERBACH 58-BU-20 blue for TPU
	81001135	MASTERBATCH 1031-BU-50 for polyamide, blue
	81001134	MASTERBATCH 1-RD-72 ZA POLIAMID for polyamide, red
	81000267	Green colour 232-GN-50 for POLIAMID
	81001315	MASTERBATCH UN2319-ORANGE
	81000916	MASTERBATCH 77-GY-50 grey PE/PP

lists the base materials and additives along with their corresponding reference numbers. To ensure clarity and avoid redundancy, the material reference numbers defined in Table 1 are used throughout

Table 2. Material combinations.

Basic Material Group	Mixture Mark (Sample No._Thickness)	Material Combination (Base Material + Additive(s))
1	S1_1.5	81001531
	S1_3	81001531
	S2_1.5	81001531 + 81001532—4%
	S2_3	81001531 + 81001532—4%
	S3_1.5	81001531 + 81000577—2%
	S3_3	81001531 + 81000577—2%
	S4_1.5	81001531 + 81000577—2% + 81001532—4%
	S4_3	81001531 + 81000577—2% + 81001532—4%
	S5_1.5	81001531 + 81001483—2%
	S5_3	81001531 + 81001483—2%
2	S6_1.5	81001295
	S6_3	81001295
	S7_1.5	81001295 + 81001119—2%
	S7_3	81001295 + 81001119—2%
3	S8_1.5	81001482
	S8_3	81001482
	S9_1.5	81001482 + 81001483—2%
	S9_3	81001482 + 81001483—2%
4	S10_1.5	81001443
	S10_3	81001443
5	S11_1.5	81002009
	S11_3	81002009
	S12_1.5	81002009 + 81001119—2%
	S12_3	81002009 + 81001119—2%
6	S13_1.5	81001810
	S13_3	81001810
	S14_1.5	81001810 + 81001119—2%
	S14_3	81001810 + 81001119—2%
7	S15_1.5	81000936
	S15_3	81000936
8	S16_1.5	81000935
	S16_3	81000935
9	S17_1.5	10223024
	S17_3	10223024
10	S18_1.5	10238024
	S18_3	10238024
11	S19_1.5	81000264
	S19_3	81000264
	S20_1.5	81000264 + 81001135—2%
	S20_3	81000264 + 81001135—2%
	S21_1.5	81000264 + 81001483—2%
	S21_3	81000264 + 81001483—2%
	S22_1.5	81000264 + 81001134—2%
	S22_3	81000264 + 81001134—2%
	S23_1.5	81000264 + 81000267—2%
	S23_3	81000264 + 81000267—2%
12	S24_1.5	81000863
	S24_3	81000863
	S25_1.5	81000863 + 81001315—2%
	S25_3	81000863 + 81001315—2%
13	S26_1.5	81001893
	S26_3	81001893
14	S27_1.5	81000917
	S27_3	81000917
	S28_1.5	81000917—48% + 81000918—48% + 81000916—2% + 81000577—2%
	S28_3	81000917—48% + 81000918—48% + 81000916—2% + 81000577—2%
15	S29_1.5	81000918
	S29_3	81000918
16	S30_1.5	81000853
	S30_3	81000853
17	S31_1.5	81000851
	S31_3	81000851
	S32_1.5	81000851 + 81001315—2%
	S32_3	81000851 + 81001315—2%

to represent the individual components of each sample.

Table 2 presents the various material combinations prepared and tested. Each combination is identified by a mixture mark, which includes a consecutive number and the sample thickness (e.g., S1_1.5—sample one, 1.5 mm thick). We grouped materials based on their base composition, and some groups explored variations in thickness (see column two) and additive concentrations (see column three).

Following the standard testing procedure described in Section 2, we classified the samples into HB, HB40, and HB75 categories. All measurement results were gathered in a table with all relevant information for each tested sample. Figure 4 illustrates the burning test process (left—implementation of burner, middle—sample flaming, right—front of the flame reaching 100 mm mark).

3.2. Classification of Test Samples by Burning Categories

We summarized the final classification of tested samples into HB, HB40, HB75, and unclassified categories in

Table 3. Classification of tested materials by burning category.

Material Category HB	Material Category HB40	Material Category HB75	Cannot Be Categorized
S8_1.5	S2_3	S1_3	S1_1.5
S8_3	S7_3	S2_1.5	S3_1.5
S9_1.5	S11_3	S3_3
S9_3	S12_1.5	S4_1.5
S10_1.5	S12_3	S4_3
S10_3	S13_1.5	S5_1.5
S13_3	S20_1.5	S5_3
S14_3	S23_1.5	S6_1.5
S15_1.5	S23_3	S6_3
S15_3	S24_1.5	S7_1.5
S16_1.5	S26_1.5	S11_1.5
S16_3	S27_1.5	S14_1.5
S17_1.5	S27_3	S28_1.5
S17_3	S28_3	S30_1.5
S18_1.5	S29_1.5	S30_3
S18_3	S29_3
S19_1.5
S19_3
S20_3
S21_1.5
S21_3
S22_1.5
S22_3
S24_3
S25_1.5
S25_3
S26_3
S31_1.5
S31_3
S32_1.5
S32_3

. We uncategorized two samples due to significant bending and sustained burning on the wire gauze, violating the EN 60695-11-10 test conditions. Figure 5 visually represents the number of materials per category. There are 31 samples in the category HB, 16 samples in the category HB40, 15 samples in the category HB75 and 2 samples that could not be categorized (N/A).

3.3. Classification of Test Samples by Category and Thickness

Table 4. Classification of test samples according to burning categories and sample thickness.

Material Thickness	Material CategoryHB	Material CategoryHB40	Material CategoryHB75	Cannot Be Categorized
1.5 mm	S8_1.5	S12_1.5	S2_1.5	S1_1.5
	S9_1.5	S13_1.5	S4_1.5	S3_1.5
	S10_1.5	S20_1.5	S5_1.5
	S15_1.5	S23_1.5	S6_1.5
	S16_1.5	S24_1.5	S7_1.5
	S17_1.5	S26_1.5	S11_1.5
	S18_1.5	S27_1.5	S14_1.5
	S19_1.5	S29_1.5	S28_1.5
	S21_1.5		S30_1.5
	S22_1.5
	S25_1.5
	S31_1.5
	S32_1.5
3.0 mm	S8_3	S2_3	S1_3
	S9_3	S7_3	S3_3
	S10_3	S11_3	S4_3
	S13_3	S12_3	S5_3
	S14_3	S23_3	S6_3
	S15_3	S27_3	S30_3
	S16_3	S28_3
	S17_3	S29_3
	S18_3
	S19_3
	S20_3
	S21_3
	S22_3
	S24_3
	S25_3
	S26_3
	S31_3
	S32_3

classifies the samples according to both burning categories and thicknesses (1.5 mm and 3.0 mm). Figure 6 visually summarizes these results, clearly indicating the number of materials per category and thickness.

This classification indicates which material mixtures consistently achieve the same burning category at specific thicknesses, highlighting groups of material mixtures such as PC Makrolon, PA6 Bergamid, PA66 Ultramid combined with RAL 7021 pigment, and Masterbatch for polyamide.

3.4. Classification of Test Samples by Material Mixture

Table 5. Classification of test samples according to burning categories and material mixtures.

Basic Material Group	Mixture Mark	Material Combination	Material Category
1	S1_1.5	81001531	Cannot be categorized
	S1_3	81001531	HB75
	S2_1.5	81001531 + 81001532—4%	HB75
	S2_3	81001531 + 81001532—4%	HB40
	S3_1.5	81001531 + 81000577—2%	Cannot be categorized
	S3_3	81001531 + 81000577—2%	HB75
	S4_1.5	81001531 + 81000577—2% + 81001532—4%	HB75
	S4_3	81001531 + 81000577—2% + 81001532—4%	HB75
	S5_1.5	81001531 + 81001483—2%	HB75
	S5_3	81001531 + 81001483—2%	HB75
2	S6_1.5	81001295	HB75
	S6_3	81001295	HB75
	S7_1.5	81001295 + 81001119—2%	HB75
	S7_3	81001295 + 81001119—2%	HB40
3	S8_1.5	81001482	HB
	S8_3	81001482	HB
	S9_1.5	81001482 + 81001483—2%	HB
	S9_3	81001482 + 81001483—2%	HB
4	S10_1.5	81001443	HB
	S10_3	81001443	HB
5	S11_1.5	81002009	HB75
	S11_3	81002009	HB40
	S12_1.5	81002009 + 81001119—2%	HB40
	S12_3	81002009 + 81001119—2%	HB40
6	S13_1.5	81001810	HB40
	S13_3	81001810	HB
	S14_1.5	81001810 + 81001119—2%	HB75
	S14_3	81001810 + 81001119—2%	HB
7	S15_1.5	81000936	HB
	S15_3	81000936	HB
8	S16_1.5	81000935	HB
	S16_3	81000935	HB
9	S17_1.5	10223024	HB
	S17_3	10223024	HB
10	S18_1.5	10238024	HB
	S18_3	10238024	HB
11	S19_1.5	81000264	HB
	S19_3	81000264	HB
	S20_1.5	81000264 + 81001135—2%	HB40
	S20_3	81000264 + 81001135—2%	HB
	S21_1.5	81000264 + 81001483—2%	HB
	S21_3	81000264 + 81001483—2%	HB
	S22_1.5	81000264 + 81001134—2%	HB
	S22_3	81000264 + 81001134—2%	HB
	S23_1.5	81000264 + 81000267—2%	HB40
	S23_3	81000264 + 81000267—2%	HB40
12	S24_1.5	81000863	HB40
	S24_3	81000863	HB
	S25_1.5	81000863 + 81001315—2%	HB
	S25_3	81000863 + 81001315—2%	HB
13	S26_1.5	81001893	HB40
	S26_3	81001893	HB
14	S27_1.5	81000917	HB40
	S27_3	81000917	HB40
	S28_1.5	81000917—48% + 81000918—48% + 81000916—2% + 81000577—2%	HB75
	S28_3	81000917—48% + 81000918—48% + 81000916—2% + 81000577—2%	HB40
15	S29_1.5	81000918	HB40
	S29_3	81000918	HB40
16	S30_1.5	81000853	HB75
	S30_3	81000853	HB75
17	S31_1.5	81000851	HB
	S31_3	81000851	HB
	S32_1.5	81000851 + 81001315—2%	HB
	S32_3	81000851 + 81001315—2%	HB

and Figure 7 display the results grouped by base material and additives, demonstrating how mixtures within the same basic material group vary in their flammability categories. Material groups 3, 4, 7, 8, 9, 10, and 17 consistently fell into the same burning category across thicknesses and additive combinations. Conversely, groups 1, 2, 5, 6, 11, 12, 13, and 14 showed varied flammability results within their respective groupings. Some mixture combinations result in a different burning category. Material groups number 15 and 16 are consistent within one basic material but have different thicknesses.

Figure 7 presents the vertical axis material burning categories and horizontal axis basic material group of samples (first and second column in Table 5). Each group of samples consists of different material composition samples.

The results show which samples are in the same burning category and as such provide valuable information for manufacturers of insulation material regarding which material and additions combinations have the same burning category result. These findings are valuable for manufacturers in determining consistent material and additive combinations that reliably achieve the desired flammability performance, optimizing material selection and reducing the need for extensive testing. The sample size per material group was too small for robust statistical analyses, limiting the applicability of traditional significance tests. Nevertheless, several empirical observations are noteworthy. Certain inorganic pigments and fillers can improve flame resistance by promoting char formation during combustion, thus creating an insulating barrier on the polymer surface. This protective char layer reduces heat transfer, slows material degradation, and limits combustible gas release, improving the overall flammability performance even without specialized flame retardant additives. Although thicker samples generally exhibit better flame resistance due to their higher thermal mass, our results indicate that minor compositional variations, particularly the type of additives used, often overshadow thickness effects. Thus, additive selection appears more critical than thickness alone in determining the HB classification.

4. Discussion

This paper presents a systematic approach to fire hazard testing aligned with the EN 60695-11-10 standard, focusing on determining the compliance of various materials and additive mixtures with standardized flammability categories. We tested 64 material mixtures using the horizontal burning test (method A) prescribed by EN 60695-11-10. These mixtures were grouped based on their primary base material, resulting in 17 distinct groups. Additionally, the results were categorized according to the sample thickness to analyze its impact on flammability performance.

The results revealed that among the tested mixtures we classified, 31 samples were in the HB category, 16 samples fell into the HB40 category, and 15 samples met the criteria for HB75. Only two samples could not be categorized under any of the specified flammability classifications. When analyzed according to thickness, the results indicated that 13 samples with a thickness of 1.5 mm achieved the HB category, 8 were categorized as HB40, and 9 fell into the HB75 category. Conversely, among samples with a thickness of 3.0 mm, 18 were classified as HB, 8 as HB40, and 6 as HB75.

A further examination of material groups with consistent base materials highlighted clear trends and deviations. Specifically, seven groups consistently achieved an HB classification regardless of thickness or additive variations. Additionally, one group consistently achieved HB40, and another consistently attained the HB75 classification. However, some material groups exhibited deviations, with certain additive combinations significantly altering their classification. These deviations provide essential insights for manufacturers, indicating which material mixtures may require additional scrutiny or specific adjustments in their formulation to maintain the desired fire safety properties.

Our results align well with findings from previous studies examining the effects of minor additives on HB flammability. Razak et al. demonstrated that inorganic fillers significantly affect flammability behaviour, confirming our observations regarding the fillers’ impact [9]. Wang et al. similarly found improved flame resistance in polyamide 6 composites with thermal conductive fillers, aligning with our findings about thickness-dependent performance [7]. Conversely, Czel et al. indicated that different fillers have varied impacts on polyamide 6 behaviour, supporting our conclusion about the unpredictability of compositional changes [15]. These studies collectively reinforce the importance of empirical evaluations when considering minor formulation variations in polymers.

The results presented here underscore the complexity of predicting flammability performance based solely on base material properties, confirming earlier studies that suggested that additive interactions could considerably influence combustion behaviour. These observations highlight the ongoing challenge in predicting material performance from limited test parameters and support arguments for broader experimental evaluations.

Future research directions should focus on predictive modelling to further reduce the need for repetitive testing. Developing accurate computational models or machine learning techniques to predict flammability categories based on specific material compositions and thickness variations could greatly enhance the efficiency of material selection and certification processes. Additionally, larger-scale tests and comparisons with real-world fire scenarios could validate the practical relevance of these standardized small-scale tests.

The findings from this study offer valuable insights for manufacturers in material selection and product development phases. By identifying material and additive combinations that consistently meet defined flammability standards, manufacturers can optimize their choices strategically to meet customer demands effectively while minimizing additional fire hazard testing requirements. Several material groups demonstrated consistent flammability classification regardless of minor additive changes or the sample thickness. Specifically, Groups 3, 4, 7, 8, 9, 10, and 17 consistently met the HB classification across all tested compositions, while Groups 2 and 16 reliably achieved HB75 and Groups 5, 14 and 15 reliably achieved HB40. In such cases, it is reasonable to recommend a “worst-case testing” approach, where only the thinnest samples with the least favorable additive composition are tested. If these worst-case variants meet the desired classification, the same classification could be assumed for related compositions within the group, thereby reducing certification costs and avoiding redundant testing. On the other hand, Groups 1 and 11 exhibited variability between HB, HB40, and HB75 classifications depending on the additive type or thickness, indicating that full testing across variants remains necessary. Ultimately, this approach enhances both flexibility and cost efficiency in material utilization without compromising product safety, thereby strengthening manufacturers’ competitiveness and compliance on global markets but not risking the safety of final product.

5. Conclusions

This study systematically evaluated 64 polymer material combinations for horizontal burning behaviour according to EN 60695-11-10. The key findings are as follows:

• Minor changes in composition, such as pigments and fillers, significantly affected the flammability classification (HB, HB40, HB75) in several material groups.
• Seven material groups showed consistent classification across all tested variations, suggesting potential for reducing redundant testing.
• Sample thickness had less influence than expected; thinner samples did not always show a worse performance.
• Manufacturers can leverage these findings to optimize material selection, streamline safety certification, and reduce testing costs by focusing on “equivalent” additive systems.

Future work should focus on predictive modelling to pre-assess the flammability performance, thus reducing the need for extensive empirical testing.