An Approach for the Accelerated Heat Aging of Test Specimens Produced Using 3D Additive Materials ^†

Abstract

In this work, an approach has been developed for the accelerated aging of test specimens with an electrical furnace for tensile strength and hardness testing, manufactured using a 3D Fused Deposition Modeling (FDM) printer. A detailed study of the climatic conditions and temperature amplitudes of the city of Sofia, Bulgaria, over a period of one calendar year was conducted. The ASTM D3045 standard was used, and numerical expressions were compiled, which guided the authors in determining the period and temperature of heating. Eight types of FDM 3D printing materials with different properties and characteristics were used in this study.

1. Introduction

Materials science is a combination of elements of physics, chemistry, and engineering [1,2]. Its study began in 1940 and consists of investigating their structure and properties at the macro level [3].

Additive 3D printing materials are synthetic polymers or composites that consist of very large molecules or macromolecules, which are made up of many repeating subunits derived from one or more types of monomers. They are increasingly used in industry and everyday life, as the characteristics of the materials vary widely. They come in different categories such as heat-resistant, high-strength, chemical-resistant, electrically conductive, flexible, and high light transmittance. Each of the 3D materials has unique properties suitable for a wide range of needs [4]. The molecular weight of each polymer is different and depends on the chain length or degree of polymerization.

Unfortunately, polymers are not as strong and resistant to external factors as metals. They are affected and deformed when exposed to heat, ultraviolet light, visible light, moisture, and liquids, and a variety of physical and chemical changes are caused in plastics [5]. The level of exposure to both temperature and time affects the type and extent of property changes that will occur. Polymers do not deteriorate with short-term exposure to high temperatures, but when exposed to high temperatures for a long time, they lose some of their properties and undergo a gradual change in physical properties. Most polymers are amorphous and undergo physical relaxation and other structural changes over time. Softer materials are more susceptible to time-related changes due to their more flexible chains. The chemical composition of the polymer, the presence and amount of oxidation-prone groups, and the amount of additives have a strong effect.

Prolonged exposure of polymers to these adverse conditions—UV radiation from the Sun—results in the destruction or deformation of the test specimen. This leads to a change in tensile strength, a change in their color and shape, and a change in molecular weight [6]. Standards have been developed by the American Society for Testing and Materials (ASTM) that define a standard practice for the thermal aging of unloaded plastics. According to [7], the means that help catalyze the process are radiation from lamps such as xenon, metal halide, fluorescent lighting tubes, and mercury. Also, the UV radiation that reaches the Earth has a wavelength of 290–400 nm. In another scientific paper by the authors of [8], it is suggested that devices capable of reproducing natural exposure to the Sun on the surface of the polymer should use wavelength thresholds of 290–320 nm. These wavelengths are important to consider when deciding which range to use to achieve the best polymer aging results. They also argue that accelerated aging is possible because photochemical processes primarily facilitate the degradation of polymers.

Accelerated aging tests can be performed under the influence of different environments. Heating can be performed using air and various lubricants, such as oil and grease, at room and elevated temperatures, for different periods of time, as carried out in [9]. In [10], the authors present a machine for the accelerated aging of test specimens with UV radiation combined with the immersion of test specimens in water for the purpose of periodically moisturizing them.

From an engineering application perspective, the effects of polymer structure are limited at low polymer content, while more significant differences in properties are observed at high polymer content. To fully exploit the advantages of SBS polymer modification, the combined influences of polymer structure, content, and aging must be taken into account according to the authors of [11].

The aim of this work is to develop an approach for aging test specimens made from different groups of filaments through 3D additive printing. Accelerated aging will correspond to exposure to UV radiation for a period of one calendar year under meteorological conditions in the city of Sofia, Bulgaria, in 2024. The test specimens will be monitored for changes in shape and color to determine whether changes in appearance have occurred.

2. Approach and Method for Determining the Accelerated Aging Period of the Test Specimens

2.1. Approach

The amount of ultraviolet (UV) radiation from the Sun that reaches the Earth’s surface, and consequently the UV index, depends on several factors [12]:

• Time of day;
• Season;
• Latitude (annual UV doses decrease with increasing distance from the equator);
• Altitude (for every 300 m increase in altitude, UV radiation increases by about 4%);
• Surface reflectance (snow reflects up to 85% of UV rays, and water reflects approximately 5–10%);
• Clouds (the amount of UV radiation that reaches the Earth’s surface is reduced by clouds depending on their thickness, density, and shape);
• Air pollution (clouds and urban smog change the amount of UV radiation that reaches the Earth’s surface).

Typically, UV radiation is strongest for a few hours around noon and is weaker in the early morning and late afternoon/evening (see Figure 1 [12]).

Figure 1 shows that the duration of the strongest intensity of sunshine is approximately 5.5 h, with the greatest intensity of radiation in summer being between 9:00 and 15:00.

Seasonal fluctuations in UV radiation reaching the Earth’s surface have large amplitudes in temperate regions, unlike populated areas near the equator.

2.2. Method for Determining the Aging of Polymeric Materials

Many standards have been developed for testing materials to determine their durability. One of them is the Standard ASTM D3045 [13], which specifies a standard practice for the thermal aging of unloaded plastics. It simulates temperature conditions under prolonged exposure to hot air alone, which are intended to be used to determine the durability and expected life of materials in order to prevent loss. If polymers are exposed to temperatures higher than their intended use for a long time, they begin to age prematurely. This aging procedure helps to predict how these materials will age over time. The standard specifies only the heat exposure procedure, and the test method and sample size are determined by the contractor. This practice can be used to compare the thermal aging of different materials at the same temperature, as well as the same materials at a series of temperature intervals. The standard recommends the use of four successive temperatures, preferably selected from

Table 1. Monthly temperature intervals.

Number of Days with Temperature Interval
Month	20–24 °C	24–28 °C	28–32 °C	32–36 °C
May	15
June	3	12	15
July		4	12	15
August		5	10	16
September	12	10	8
October		4
Total days	30	35	45	31

(Suggested Temperatures and Exposure Times for the Determination of Heat Aging of Plastics) of his content. The lowest temperature should produce the desired level of property change or product damage in approximately nine to twelve months. The next higher temperature should produce the same level of property change or product damage in approximately six months. The third and fourth temperatures should produce the desired level of property change or product damage in approximately three months and one month, respectively.

3. Determination of the Duration of Temperature Heating

Based on the factors that are decisive for accelerated aging, the geographical location, altitude, and seasons were selected to determine the period of temperature heating of the test bodies. The location where the experiment was conducted is the capital of Bulgaria—the city of Sofia, with an average altitude of 550 m. The period of the entire calendar year 2024 was monitored, which means that all seasons of the year are covered—spring, summer, autumn, and winter. Due to the low temperatures of the autumn–winter and spring–winter seasons, the months January, February, March, April, November, and December were not taken into account. The temperatures during this period vary from −5 to +20 °C, based on information provided by the National Institute of Meteorology and Hydrology (NIMH) [14].

The remaining months from May to October 2024 are tracked in detail through the measured temperatures by the NIMH and summarized in the 2024 Monthly Hydrometeorological Bulletin [15] for each of the months.

From the Monthly Bulletin of the NIMH [14], the Air Temperature graphs for the months of May–October were used. Figure 2 presents the air temperature in May using the maximum temperature reached each day, depicted with a red line. Several groups with temperature intervals were developed, with the minimum measured temperature being 20 °C and the maximum being 36 °C:

• Group 1: 20–24 °C;
• Group 2: 24–28 °C;
• Group 3: 28–32 °C;
• Group 4: 32–36 °C.

Temperatures that are lower than 20 °C are not taken into account because they are very low and below the temperature under normal conditions. The data are recorded in Table 1 for each of the months, summarizing the number of days from all temperature intervals.

An analysis was also conducted on the number of hours of sunshine for each of the months, with the information again taken from the NIMH. The number of hours of sunshine each day was summarized for each month, and the arithmetic mean was found. Figure 3 presents a graph of the amount of sunshine in hours in the month of June 2024 in the city of Sofia. Such an analysis was conducted for all months from May to October and is described in

Table 2. Sun heating (in hours) for the months May–October 2024 in the city of Sofia.

Month	Number of Hours of Sun Heating, h	Average Sun Heating per Day, h
May	229	8.80
June	346	11.52
July	383	12.35
August	307	9.9
September	215	7.16
October	237	7.64

.

Based on the summary data from Table 1 and Table 2 and the minimum daily temperatures for each period, the period of thermal heating of the test bodies was determined. Due to the fact that the air temperature is not the same throughout the day, it is necessary to calculate all temperature intervals. According to Figure 1, it is assumed that the highest temperatures last 5.5 h of the day, and according to the duration of sunshine during each interval, the number of hours was calculated.

According to the following numerical expressions, the adjusted number of hours (nh) for all temperature groups was calculated:

nh₁ = Tmax(group) = (5.5 h × number of days (group))

(1)

nh₂ = Tmax(group n − 1) = (Sunlight Duration − 5.5)/2 × number of days (group)

(2)

nh₃ = Tmax(group n − 2) = (Sunlight Duration − 5.5)/2 × number of days (group)

(3)

The number of hours thus obtained from each group was combined.

Table 3. Equalized temperature intervals for all temperature groups in hours and days.

Group	Number of Hours with Temperature Interval
	20–24 °C	24–28 °C	28–32 °C	32–36 °C
1	165
2	87.5	192.5
3	146.25	146.25	247.5
4	62	70.15	70.15	108.5
Total hours	464.25	409.30	318.05	108.5
Total days	19	17	13	4.5

describes the adjusted temperature intervals from all temperature groups, calculated in hours and days, according to Equations (1)–(3). Group 1 (20 °C ÷ 24 °C) is not included in the calculations because it is equal to the temperature under normal conditions (room temperature).

According to the standard for the accelerated aging of polymers, the test bodies must be heated from a temperature of at least 50 °C, and the data at other temperatures were also compared. However, due to the low melting point of the test bodies, we chose to heat them at 50 °C, and the resulting duration for all groups is equal to the selected temperature, according to the standard described in point 2.2. Thus, the total duration for thermal accelerated aging, corresponding to one year of exposure to the Sun in the city of Sofia, is 11 days at a temperature of 50 °C.

4. Experimental Setup

To conduct the study, test specimens were used, Figure 4, which were manufactured using 3D printers, Tevo Tornado and Ultimaker C5, with FDM technology. Test specimens of the following types of materials were subjected to aging: PLA, PLA Wood, PETG, ASA, PC, PA6, FilaFlex SEBS, and CPE HG100, manufactured by 3D Jake, Paldau, Austria [16]. Different types of materials were selected to analyze the resistance of all of them for 1 year, after exposure to solar heating. In a previous work by the authors [17], an analysis of the printing parameters was performed, and the same printing characteristics were used for all test specimens: printing direction xy, 30% infill, and Geroid infill style. The test specimens that were prepared for accelerated aging have a shape suitable for testing the tensile strength of 3 pcs. of material and the hardness of 1 pc. Their overall dimensions are as follows: (1) Tensile strength test specimen: L—150 mm; b—20 mm; h—4 mm, (Figure 4a) and (2) hardness a = 60 mm; b = 60 mm; h = 6 mm (Figure 4b).

The test specimens were heated in an electric resistance furnace with a contact mercury thermometer with a heating range of 0–200 °C (Figure 5). The connected contactor turns the heaters on and off when necessary to maintain the set temperature of 50 °C. The furnace operates at a power of 1700 W at 220 V. In order to be able to recognize the samples, they were placed on rice paper with their names written on them.

5. Experimental Results and Discussion

After the specified heating time of the test specimens of 11 days at an air temperature of 50 °C, an organoleptic analysis was performed. Figure 6 shows all test specimens after the accelerated aging process. The images cannot give a complete picture of the behavior of the test specimens; for this reason, the observed changes in their shape are described in

Table 4. Deformation of specimens after the accelerated aging test.

Filament	Presence of Deformation
	Specimen for Hardness	Specimen for Tensile Strength
PLA	Yes	Yes
PLA WOOD	Yes	Yes
PETG	No	No
PC	No	No
ASA	No	No
PA6	Yes	No
CPE HG100	No	No
FilaFlex SEBS	Yes	Yes

.

The smaller test specimens—those for measuring hardness—are more susceptible to deformation, unlike the longer test specimens for testing tensile strength. In some of the materials, distortions are observed at the edges of the square specimens (Figure 6a). In those shown in Figure 6b, slight deformations are also observed, but they are much less significant. But after measurement of the test specimens for tensile strength, shrinkage in the length of some of the materials was observed: PLA—4 mm, PLA Wood—2 mm, and Fila Flex SEBS—1 mm.

In the hardness test specimens, the greatest distortion was observed in the PLA material, with milder deformations in the PLA Wood, PA6, and FilaFlex SEBS materials, and in the tensile strength test specimens—PLA, PLA Wood, and FilaFlex SEBS. It is not surprising that the PLA and PLA Wood materials do not withstand prolonged exposure to heat. They are from the group of easy-to-print and composite materials containing PLA. The PA6 material is from the group of wear-resistant materials, which contains an additive to reduce shrinkage. However, during the testing period, surface distortion occurred in the lower layers.

FilaFlex SEBS is from the group of flexible rubber-based materials. In terms of characteristics, it also has good resistance to UV radiation, but the results show distortion in the lower layers of the base of the test specimen. After the accelerated aging of the test bodies produced from different colors, no change in the shades was observed.

6. Conclusions

In this work, an approach for the accelerated aging of test specimens for testing hardness and tensile strength, obtained through 3D printing using additive materials, has been developed. The test specimens are made of the following materials: PLA, PLA Wood, PETG, ASA, PC, PA6, FilaFlex SEBS, and CPE HG 100 with 30% infill and a Geroid grid. The most important factors that determine the aging period are as follows: Time of day; season; latitude; altitude; surface reflectance; clouds; and air pollution. The location chosen for the study is the city of Sofia in Bulgaria, and the aging period is one calendar year. The temperature limits and hours of sunshine for the specified location have been taken into account, from which 11 days of heating of the test specimens at a temperature of 50 °C were determined. The results show the greatest distortion and deformation in the two test specimens made of PLA material. Less distortion was observed in the hardness test specimens from the materials PLA Wood, PA6, and FilaFlex SEB, and shrinkage in the tensile strength test specimens of the materials PLA Wood and FilaFlex SEBS was measured. Despite their UV resistance, the materials, PA6 and FilaFlex SEBS, exhibit slight distortion, which is most likely due to the lower density of the test specimens. The remaining test specimens, made from the materials PETG, ASA, PC, and CPE HG 100, have no changes in their appearance after exposure to accelerated aging for one year. It was also found that, in addition to the composition of the test body, the shape of the 3D-printed part also has an influence.

7. Future Steps

In future research, the aged tensile strength and hardness test specimens will be examined to determine changes in their physical properties. Thus, in addition to the behavior of the parts exposed to solar radiation, information will also be available about their strength and toughness characteristics.