Investigating the Possibilities of Using a Handheld-FTIR Equipment to Characterize Thermal Aging of Rubbers

(1)

IN

DEGREE PROJECT ENGINEERING CHEMISTRY, SECOND CYCLE, 30 CREDITS

,

STOCKHOLM SWEDEN 2020

Investigating the Possibilities of

Using a Handheld-FTIR Equipment

to Characterize Thermal Aging of

Rubbers

Under supervision of Simon Pettersson, Per Reinholdsson

and Mikael Hedenqvist

ANTONIA TENGBOM

KTH ROYAL INSTITUTE OF TECHNOLOGY

(2)

1 Abstract

Element Materials Technology in Linköping is an independent material testing company that performs testing of materials to several big sectors, such as the Swedish Armed Forces and the aerospace industry. There is of great importance to characterize aging of materials to ensure material performance. Element Materials Technology recently invested in a handheld FTIR equipment and it was of interest to see if this equipment might be an additional analysis technique to characterize aging of rubber materials. A nitrile butadiene rubber and two natural rubbers were thermally aged in 50°C and 70°C for six weeks. The hardness of the rubbers increased when the rubbers were thermally aged. The compression set decreased for the aged samples as well as the tensile strength and the elongation at break. An investigation of possible methods for collecting migrated additives on the rubber surface was performed, using two different solvents and a stationary FTIR equipment. More research needs to be performed to exclude the possibility that the solvents affect the material. Universal Attenuated Total Reflectance – Fourier Transform Infrared Spectroscopy, UATR-FTIR onto the rubber surface showed in some cases changes in the spectrums between unaged and aged samples. However, it could not be concluded weather the changes occurred due to migrating additives or due to changes in the backbone polymer. Four interfaces to the handheld FTIR equipment were investigated and an ATR interface gave best results. An analysis method was developed for the handheld FTIR equipment and the spectrums from the handheld FTIR were similar to spectrums from UATR-FTIR (stationary). It could be concluded from Micro Chamber analysis that volatile organic matters were emitted at elevated temperature. Thermogravimetric analysis could detect the relative composition of the rubbers. Further it could be concluded that the mechanical properties were affected by the thermal treatment. This study could however not establish a correlation between FTIR signals and the results seen from the other analysis. The fact that differences could be detected in the FTIR spectrums before and after aging could indicate that the FTIR analysis technique can possibly be used as an analysis method. However, a more thoroughly investigation needs to be performed before using this technique.

(3)

2 Sammanfattning

Element Materials Technology i Linköping utför materialutredningar åt en mängd olika typer av företag, där en stor sektor är den svenska Försvarsmakten och flygindustrin. Materialutredningar är essentiella för att säkerställa att rätt material används för rätt applikation. Element Materials Technology investerade nyligen i en handhållen FTIR utrustning. Det finns förhoppningar om att den nya utrustningen kan användas som ett komplement för att ålderskarakterisera gummimaterial. Ett nitril-butadien-gummi och två stycken naturgummin var termiskt åldrade i 50°C och 70°C i sex veckor. Hårdheten på provkroppar ökade efter den termiska åldringen. Sättningen, draghållfastheten och töjningen minskade för proverna åldrade i värme. En metod för att samla upp migrerade additiv från gummiytan utvecklades och undersöktes, genom att använda två olika lösningsmedel och en stationär FTIR utrustning. Ytterligare utredningar behöver däremot genomföras för att utesluta ifall de två lösningsmedel påverkar gummimaterialet och inte bara samlar upp additiv från gummiytan. UATR-FTIR visade på skillnader i spektrum mellan icke-åldrade och icke-åldrade gummimaterial. Däremot kunde det inte avgöras om skillnaderna i spektrumen berodde på migration av additiv till gummiytan eller på förändring i huvudpolymeren. Fyra stycken olika tillbehör till den handhållna FTIR utrustningen prövades, där ATR-tillbehöret gav bra spektrum. En analyseringsmetod utvecklades för den handhållna FTIR-utrustningen och spektrum från dessa analyser gav liknande spektrum som spektrum från UATR-FTIR (stationär). Analys med Micro Chamber visade att flyktiga organiska ämnen lämnade gummimaterialet vid förhöjd temperatur. Termogravimetrisk analys, TGA visade förhållandet mellan gummimaterialets komponenter både före och efter åldring. Studien visade också att de mekaniska egenskaperna påverkades av den termiska åldringen. Analysmetoden FTIR kunde detekterad skillnad i spektrum före och efter åldring av gummimaterialen. Studien kunde däremot inte etablera ett samband mellan dessa FTIR-signaler och de övriga analysresultaten. Det kan däremot inte uteslutas att ett sådant samband finns. Ytterligare studier behöver utföras för att undersöka detta.

(4)

3 Acknowledgements

I would like to thank all the people at Element Materials Technology in Linköping who helped me during my master thesis. Firstly, I would like to thank my nearest supervisors at Element Materials Technology, Simon Pettersson and Per Reinholdsson who helped me during the whole project. Further, I want to thank Birgitta Engström for her generosity to organize so that I could use their lab-equipment and also for all her help in the lab. Of course I would like to say a big thank you to all at Element Materials technology who helped me and for the nice atmosphere I was met by. Thank you also Ulrika Conte who made it possible for me to perform my mater thesis at the

(5)

1

4

5 Introduction

Element Materials Technology in Linköping is an independent material testing company that provides testing and consulting in variety of sectors such as the automotive, aerospace sector and to the Swedish Armed Forces. Material testing can involve testing the suitability and performance of a specific material in a specified environment but it can also involve investigating material failure. In some cases there are many involved parameters and an extensive investigation is necessary to find the cause of the failure.

There is of great importance to characterize and evaluate the aging of materials in order to find the correct material in the proper service environment. Rubbers are widely used in various applications, were the aerospace industry is one big sector. Rubbers used in these sectors exhibits a great variety of conditions, for example heat, UV-light, oils, fuels, ozone and mechanical load, which will affect the material during its life-span. It is therefore important to characterize the aging of the materials to ensure safe usage. Rubbers can be characterized with various methods, where some of the mechanical tests that Element Material Technologies performs are hardness, compression set and tensile properties. These tests can give information about the mechanical properties of the material. The methods require preparations of test pieces before testing and a laboratory test environment. In addition, these methods only give a value of the material property but do not tell exactly what chemical changes that have happened in the material. FTIR is a powerful method that can give information about the chemical bonds in a material. From that it might be possible to see the changes that can occur in a material as it ages. Further it might be possible to see any migrating additives to the surface using the FTIR method. Migrating additives might have a negative influence on the original performance of the material and can possibly deteriorate compatibility with other materials. It is of interest to see if the mechanical measurements can be complemented by the FTIR analysis method. Further, another interesting analysis method, a Micro Chamber technique, might also be able to support possible aging mechanisms by detecting volatile organic compounds that leaves the material, which possible also contributes to the change in performance of the material.

(8)

4

6 Objectives

The overall aim of this project was to investigate the potentials of the Agilent 4300 handheld FTIR equipment to characterize the aging of rubbers. The aim can be broken down to several sub-questions accordingly:

The aim was to:

 Investigate if the handheld or the stationary FTIR equipment could detect any changes in the backbone polymer.

 Investigate if migration of additives to the rubbers surface caused by thermal aging could be detected by the stationary FTIR equipment.

 Investigate if migration of additives to the rubbers surface caused by thermal aging could be detected by the handheld FTIR equipment.

 Investigate the capacity of the Micro Chamber technique to detect volatiles that leave the rubbers during thermal treatment.

 Investigate if the thermal stability and the composition of the rubbers were affected by thermal aging.

 Analyze how the thermal aging affected the mechanical properties of the rubbers. Some limitations were drawn in this project, which were:

 Two types of materials were investigated. The material chosen were nitrile butadiene rubber, NBR and natural rubber, NR.

 Only thermal aging were chosen.

The motivation to the limitations can be read under paragraph 8.

7 Literature research

7.1 Thermoplastics, thermosets, thermoplastic elastomers and rubbers

Polymers are the building block for all thermoplastics and thermosets. A polymer is a big molecule built up by many smaller units called monomers. A polymer can be either a homo-polymer, consisting of one type of monomer or a co-polymer, consisting of two or more types of monomers. Depending on the monomers building up the polymer, the final material will have different properties [1].

Elastomers can be classified into two groups, thermoplastics and thermosets. The difference is that a thermoplastic can be remolded when heated and cooled down, whereas a thermoset is chemically crosslinked and can therefore not be reshaped by heating and cooling [1].

(9)

5

7.2 Rubber synthesis and ingredients

7.2.1 Rubber compounds

Synthesis of rubbers and vulcanized rubbers includes a polymer and additives. Additives are essential to achieve certain material properties. Some types of additives that are used are: fillers, plasticizers, vulcanization agents, protective additives and additives to ease processing [2], [3]. The vulcanization process and the additives will be further discussed in the upcoming sections.

7.2.2 Vulcanization process

Rubbers are vulcanized, which is a process were chemical crosslinks are induced in between the polymer chains [2]. The vulcanization takes place thanks to a vulcanizing agent, for example sulfur, that links the molecular chains together [2], [4]. Sulphur as a vulcanization agent introduces sulphur links in between the polymer chains. The temperature during the vulcanization can vary between 140°C and 250°C [2], [4]. During sulphur-vulcanization, activators, accelerators and retarders are used. Common activators are zinc oxide and stearic acid. Further, a variety of different accelerators are used such as thiazoles and sulfonamides. The retarders that are used are commonly benzoic acid, salicylic acid and cyclohexyltioftalamide [2].

7.2.3 Additives in rubbers

Various additives are used in rubbers, for example, accelerators, age resistors, additive to ease the processing, reinforcement, fillers, pigments, colors and several others [2], [3]. Plasticizers are incorporated during processing, for example to ease processing. In natural rubber (NR) and styrene butadiene rubber (SBR) typically mineral oils are used as plasticizers whereas esters are used in nitrile butadiene rubber (NBR) [2]. However, esters are sensitive towards hydrolysis [2] which can be of great concern since this degrades the polymer backbone. Various other plasticizers are also incorporated into the rubber [2].

Antioxidants are added to prevent rubber oxidation. Common antioxidants are amines and phenols and their derivatives. Antiozonants can also be added, for example paraffin waxes and microcrystalline waxes. These waxes are hydrocarbon chains, either straight or branched. The waxes protect the rubber by migrating out to the surface and thereby hinder the reaction of ozone to the double bonds in rubber, as ozone makes the rubber crack upon tension [2].

Fillers are added to decrease the cost of the rubber product and/or to improve the material properties. Fillers that improve the material properties are for example carbon black and silicon dioxide [2].

Carbon black is a widely used additive for rubbers and it acts as pigment, UV-stabilizer and increases the material strength. In addition to this, it increases the hardness and compression set, but on the other hand, it decreases the elongation at break and the swelling in solvents [2]. Carbon black is produced by partial combustion of oils or organic matter whereby it will mainly consist of carbon, 96 to 99.5 %, but also of traces of oxygen, hydrogen, nitrogen and sulphur [2], [5]. Carbon black can appear in three different ways; as particles, aggregates or agglomerates, where the type and the amount of carbon black affect the final properties of the rubber [2].

7.2.4 Manufacturing

The synthesis of rubbers includes mixing the polymer with the additives in either a closed or an open system. The vulcanization agent can either be added in the blending step or later. If including it in the blending step there is a risk for pre-vulcanization. The mixed compounds are then shaped and thereafter vulcanized. The vulcanization can be performed for example in an autoclave, by compression molding or by injection molding [2], [6].

7.3 Commercially used rubbers

(10)

6 7.3.1 Natural rubber, NR

Natural rubber is built up by isoprene monomers, the structural unit can be seen in figure 1 [1], [2], [4]. Latex can be collected from the tree Hevea Brasiliensis, which is the feedstock for natural rubber. Ones the rubber is subjected to strain it can crystallize [2], [4]. It has a low Tg (glass transition temperature) which made it as a major use in tires. Also, it is used as dampers and for tubing. However, the lack of resistance towards oils and fuels limits its usage [2], [4].

7.3.2 Styrene butadiene rubber, SBR

Styrene butadiene rubber, figure 2, is a co-polymer with a varying polystyrene content of 23 to 40 %. The styrene part contributes to a rubber with higher damping than NR and further the material also has higher heat resistance than NR. Drawbacks with SBR are the lack of resistance towards oils and ozone. Applications for SBR include tires, flooring, shoes etc. since it was produced to replace natural rubber [2], [4].

7.3.3 Nitrile butadiene rubber, NBR

Acrylonitrile-butadiene rubber is build-up by two monomers, butadiene and acrylonitrile, see figure 3. The acrylonitrile content can vary in the material, from 18 to 50 %. The nitrile block in the backbone contributes to strong secondary bonding, leading to, for example, higher hardness and greater resistance to heat, oils, fuels and chemical resistance [1], [2], [4], [7]. On the other hand, the unsaturation, lead to a lower resistance towards aging [7]. The acrylonitrile block leads also to a lower elongation at break. Rubber containing

high amount of acrylonitrile can be used up to temperatures of 90°C to 100°C. The Tg of NBR rubber varies due to the relative content of the two monomers, where poly(acrylonitrile) have a Tg of 90°C whereas polybutadiene has a Tg of -100°C. NBR rubber is widely used, and common sectors include vehicles and engineering industry. Products of NBR are commonly used in O-rings and sealing materials [2], [4].

7.4 Aging of rubbers – chemical aging

Rubbers are subjected to varying conditions in their working environment. Possible environments that rubbers come in contact with are oxygen, heat, UV-light, ozone, oils and fuels. All these environments affect the material properties in different ways. Rubbers in aerospace industry can for example come in contact with various jet fuels but also in combination with high temperatures or mechanical load.

7.4.1 Oxygen environment

Firstly, in an oxygen environment there is a risk for oxidation. Oxidation can result in crosslinking and scission of polymer chains [7], [8], [9]. Whereas NBR becomes harder as it is oxidized, NR tends to become softer in the beginning of the aging but at a longer exposure to oxygen it will become harder [2]. Rubbers containing double bonds are sensitive to oxidation. The reaction with oxygen will be faster at higher temperatures, thereby the combination of oxygen and a hot environment will be even more deteriorating for the material stability [2]. Oxidation of polymers is a radical reaction, why it contains the three stages initiation, propagation and termination. The outcome of the oxidation will thereby be either chain scission or crosslinking [9]. Except from increasing the reaction speed, heat from the surrounding will also increase the polymer backbone mobility. This can affect the additives added to the rubber during processing. Since some additives are not chemically linked to the polymer backbone, the diffusion will be facilitated as the temperature is increased due to higher mobility of the polymer backbone [2]. The phenomenon of additives migrating out to the surface is called blooming and is more thoroughly discussed in section 7.5. The loss of additives can change the material properties even though no chemical reaction have occurred [9].

Figure 1: Isoprene monomer, the building block for natural rubber.

Figure 3: The building block for nitrile butadiene rubber, NBR. To the left: butadiene monomer. To the right: acrylonitrile monomer.

(11)

7 7.4.2 UV and ozone exposure

When it comes to the effect of light, the rate of oxidation will increase in the exposure of UV-light. However, since many rubbers contain carbon black, it is partly protected against degradation. Additionally, titanium dioxide protects against degradation from UV-light [2]. Further, ozone also affects the rubbers. Ozone tends to induce crack in the material as it is strained. To prevent this, waxes are commonly incorporated, which are diffusing out to the rubber surface. The mechanism of ozone protection is further described in the section 7.5. A surface defect that can happen onto light materials is that the surface becomes frosty [2].

7.4.3 Contact with chemicals

Fuels and chemicals can affect the rubber in such way that the liquid swell the rubber, induce migration of additives or initiate chemical reactions with the rubber components [10]. When the type of material is changed for a component it is important to consider if the new material is compatible with all the other materials involved. A previous study made at Element Materials Technology, [11], studied the compatibility of O-rings of a jet fuel with bio-based origin and compared this with a conventional and a standard test fuel. It was seen that in some of the tests the compatibility of the two fuels with the rubber behaved differently than the standard test fuel [11].

Another work, [12], studied the effect of oils on nitrile rubber. Liu et al [12], thermally aged rubber samples in a lubricating oil and its base oil and compared the result to the aging profile of rubber only aged in heat. It was seen that the migration of additives from the rubber were more slowly decreased when the samples were aged in oils than in air. They could conclude that the oils had three evident results onto the aging of the rubber. Firstly, the oil hindered the oxygen to diffuse into the rubber, hence slowing down the oxidation process. Then, it also slowed down the migration of additives from the rubber to the oil, due to this layer formation. Moreover, another effect of the aging in the oil was the fact that the oil oxidized itself, consuming oxygen, namely decreasing the extent of the oxidation of the rubber. Finally, in the case for the full oil, the additives migrated into the NBR sample due to good compatibility between the two phases and therefore increasing the aging rate [12].

7.4.4 Thermal treatment and previous research

The thermal stability of NBR has extensively been studied before in various articles [7], [13]. It has been seen that there are three stages when NBR is thermally aged [7], [12]. Firstly, there is a loss of additives from the rubber. Secondly, more crosslinks are being created and lastly oxidation will decrease the chain length [7]. The thermal aging was investigated in [7] where NBR rubber was exposed to 65°C for up to 60 days. As the rubber was aged, the elongation at break decreased and the tensile strength firstly increased but later decreased. This was explained by the increase in crosslinking density causing the movement of the chains to decrease and leading to stress concentrations [7]. The ATR-FTIR showed nine peaks for the unaged sample, at 3350 cm-1 (N-H), 2918 cm-1 (methylene), 2848 cm-1 (methylene), 2233 cm-1(nitrile bond), 1730 cm-1(C=O), 1660 cm-1 (C=C), 1580 cm-1(additives), 1433 cm-1 (-CH2-) and 964 cm-1 (C-H). There was a change in the spectrum when the rubber was aged, which resulted in a decreased of the peak at 1730 cm-1 (consumption of carbonyl groups or loss of plasticizers), whereas the peaks at 3350 cm-1, 2918 cm -1

and 2848 cm-1 increased. It was seen that thermal aging caused hydroxyl groups to be formed [7]. A previous study, [14], studied the migration of additives from NBR during thermal aging. The thermal aging was set to 60°C and 80°C and the air was analyzed using a headspace-gas chromatography-mass spectrometer, headspace-GCMS. A higher temperature gave rise to higher content of additives. 2-butoxyethanol and N-methyl aniline was seen for both aging conditions, and dimethyl urea and tetramethylurea was seen at 80°C. The oxidation of the rubber was larger at 80°C than at 60°C. The peak at 2240 cm-1 corresponding to the nitrile group, decreased with time [14].

(12)

8

7.6. It was seen in the natural rubber bearing used in Pelham Bridge that the oxidation aging only reached in 50 mm into the damping and that this resulted in an increase of the shear stress of 10 % [15]. It was also concluded that the most prominent aging environment, that changed the properties most, was the thermal aging [15].

7.5 Blooming phenomena – physical aging

If the previous paragraph explained aging cause by chemical reactions of the polymer backbone, the upcoming section rather deals with a physical aging, involving for example the blooming phenomena.

7.5.1 Types of blooming

Blooming is a phenomenon that can be divided into four different types; true, modified, pseudo bloom and surface contamination [16]. Firstly, a true blooming can evolve when substances, e.g. additives, crystallizes when the temperature is decreased after vulcanization. The crystals formed in the rubber will have a higher solubility due to pressure on them. The crystals on the surface are not affected by this pressure. Therefore, diffusion will even out the difference in dissolved matter and there will be an outflow of matter to the surface. Sulphur, zinc dithiocarbamates, mercaptobenzothiazole, zinc mercaptobenzimidazole and waxes, are commonly some, substances that tend to bloom [16]. The mechanism of modified bloom can be explained by the example of paraphenylenediamine, PPD. PPD tends to diffuse out from the bulk to the surface and there it will acts as an antiozonants. When it reacts with the ozone, a protective layer will be formed. Another compound that can create a modified bloom is zinc stearate [16]. Pseudo bloom does not consist of any substance that migrates out to the surface, but rather a degradation of the rubber itself. This happen more often to rubbers with light color [16].

Waxes are added to act as a protection against ozone degradation. Waxes have a melting temperature that varies between 30°C and 79°C, however, mostly around 50°C to 60°C [17], [18], [19]. When waxes are mixed in during the production of rubber, they are well mixed with the other rubber compounds, however, when the temperature is decreased, they tend to diffuse out to the surface of the rubber [17], [18], [19]. At the surface the waxes hinders the ozone attack [18]. When analyzing the bloom, one can either analyze the rubber containing the bloom or one can take away the bloom from the surface. However, when the rubber contains carbon black, it is commonly better to analyze only the bloom, since carbon black limits the use of IR-analysis techniques. The removal of the bloom can be done either by a wet or a dry technique. A wet technique can involve using a suitable solvent whereas the dry method can involve using wipes, a razor blade or a tape [16].

7.5.2 Previous research on blooming phenomena

In the study made of [17], the migration of paraffin waxes to SBR rubber surface was investigated. The study aimed to investigate how the amount of paraffin wax and zinc stearate on the surface changed as the temperature was increased. They heated the samples in temperature ranging from 40°C to 90°C during 15 hours and saw that under the melting point of the waxes, higher temperatures lead to lower amount of disappeared waxes from the surface. However, at higher temperatures, wax removed from the surface. Zinc stearate migration was seen to be favored when the temperature was increased above 90°C [17].

Another study, performed by Torregrosa et al. [18], investigated how the reactivation temperature affected the migration of paraffin wax to a SBR rubber. The reactivation process involved heating the rubber at a temperature of 40°C to 170°C and applying IR radiation during 10 seconds. In the lower temperature range (40°C to 70°C), they saw that a higher amount of waxes was achieved. In the range of 80°C to 120°C, some of the waxes could not be found on the surface. While at the highest range, 130°C to 170°C, there was a loss of both wax and zinc stearate. It was also seen that the zinc stearate migrated to the surface at a temperature of 90°C to 100°C [18].

7.6 Accelerated aging and Arrhenius equation

(13)

9

can be extrapolated to normal usage temperatures. Arrhenius equation, equation 1, involves k

(time unit -1), the rate of the reaction, Ea (J mol-1), the activation energy of the specific reaction, R

(8.314472 J K-1mol-1) the gas constant, T (K) the temperature in Kelvin and a pre-exponential

factor A [9], [20].

𝒌 = 𝑨 ∗ 𝒆

−𝑹∗𝑻𝑬𝒂 ₍₁₎

When using Arrhenius equation it is important to consider its assumptions and limitations. Firstly, one assumption is that the activation energy and the rate constant, k, is assumed to be constant. This is however, not always the case [9]. In the case of the rate constant, it can change due to diffusion limited oxidation, DLO. This problem arises if the consumption of oxygen is faster than the rate of oxygen that reenters the material. This can give misleading results since the bulk properties can be different from the surface properties [9]. Not only can the oxidation of polymer chains be evaluated by Arrhenius equation but also the migration of plasticizers [21].

Element Materials Technology performs accelerated aging studies onto various different materials. A rule of thumb that they use is that an increase of 10°C, results in a doubling of the reaction speed. Element Materials technology have used the analysis method micro calorimetry to receive more information, for example about the acceleration factor, that can be used to design the aging experiments. This rule of thumb is however an assumption that needs to be considered when analyzing the results. If an elevated temperature is used in the experiment, the time the samples are exposed to this temperature can be calculated to what time it corresponds to, at a reference temperature [22].

7.7 Current methods for characterizing aging of rubber materials (at Element Materials Technology)

Rubbers are commonly characterized with the mechanical measurement’s hardness, compression set, tensile properties and change in mass and volume. To evaluate a material and to predict its properties, accelerated aging can be performed. The mechanical properties are then measured before and after the aging to see the effect of the aging environment or the mechanical properties are compared to unaged reference samples. Accelerated aging is a widely used method to predict aging of materials in a shorter time. Accelerated heat aging is a common test method and has been extensively studied for many rubber materials. Performing heat ageing tests onto rubbers is an easy way to estimate long time behaviors and to predict properties at elevated temperatures. Two methods of heat aging can be made, an oven method, usually set to 70°C or 100°C, or an oxygen pressure chamber set at 70°C [10]. As described above, oxidation is one of the aging phenomena [8], [10]. Oxidation of the rubber is limited by the rate of diffusion of oxygen into the material, and therefore, the rubber can have been aged differently at the surface and the bulk of the material [10].

7.7.1 Change in weight and volume

(14)

10 ∆𝒎𝟏𝟎𝟎= 𝒎𝒊−𝒎𝟎 𝒎𝟎 × 𝟏𝟎𝟎 (2) ∆𝑽𝟏𝟎𝟎= ( 𝒎𝒊−𝒎𝒊,𝒘+𝒎𝒔,𝒘 𝒎𝟎−𝒎𝟎,𝒘+𝒎𝒔,𝒘− 𝟏) × 𝟏𝟎𝟎 (3) 7.7.2 Hardness

The hardness of a material is defined as the resistance to plastic deformation by indentation of the test equipment into the test piece. The test can be performed according to ASTM D2240 where the result is reported in °Shore A [24]. The change of the hardness in percentage before and after aging can be calculated from equation 7.

7.7.3 Tensile measurements

Tensile measurement is a common method to measure the tensile strength, elongation at break and Young’s modulus of the material. The tensile strength gives a value of the maximum strength a sample can take before it breaks, and the unit is Pascal (N/m2=Pa). The elongation at break is comparably, how much the sample can be stretched before it breaks, and is unit less. Stiffness or the Young’s modulus is the resistance to elongation and is also expressed in Pascal (Pa). The measurements can be performed according to ISO 37 [25]. The change in respective property can be calculated from equation 7. In equation 4 the tensile strength is calculated from, F the force applied to the test piece (N), A is the area where the force is applied, w is the width and t the thickness. In equation 5, the elongation at break is calculated from, lb the length of the dumb-bell at break, and l0 the initial length of the dumb-bell.

𝝈 =𝑭 𝑨= 𝑭 𝒘×𝒕 (4) 𝜺𝒃= 𝒍𝒃−𝒍𝟎 𝒍𝟎 × 𝟏𝟎𝟎 (5) 7.7.4 Compression set

In compression set testing, the materials ability to return to its original shape after compression is measured. This gives an indication of how the elastic properties are affected [10], [26]. The compression set, CS, can be calculated from equation 6, where t0 is the initial thickness, tr the thickness after the measurement and ts the thickness of the spacers used during the experiment [10]. The measurements can be performed according to ISO 815 [26]. The change in compression set between an unaged and an aged sample can then be calculated from equation 6.

𝑪𝑺 = 𝒕𝒐−𝒕𝒓

𝒕𝟎−𝒕𝒔× 𝟏𝟎𝟎 (6)

7.7.5 Change in material property before and after aging

The percentage change in, weight, volume, hardness, tensile properties and compression set can be calculated from equation 7, where x1 is the property after the aging and x0 is the initial property. The change can then be expressed in percentage, equation 7.

∆𝑪𝒉𝒂𝒏𝒈𝒆 𝒊𝒏 𝒑𝒓𝒐𝒑𝒆𝒓𝒕𝒚 =𝒙𝒊−𝒙𝟎

(15)

11 7.8 Fourier Transform Infrared Spectroscopy, FTIR – stationary and handheld

7.8.1 General information about FTIR

FTIR analysis is a powerful technique that can be used for analyzing solids, liquids and gases. By analyzing functional groups, identification of material and substances can be made by comparing characteristic peaks and spectrum available in spectral libraries [27].

The basic principles of infrared spectroscopy are that a light beam is subjected to the sample. The constituent molecules have a certain vibrational frequency. This vibrational frequency will be enhanced when light hits them and the energy of that frequency is absorbed and can be measured. The results are plotted in a graph where the transmittance or absorbance is plotted against the wavenumber. Wavelengths commonly used are 4000 to 600 cm-1 [28].

Attenuated Total Reflection FTIR, ATR-FTIR, is a similar technique but instead of using transmission mode, it uses total reflectance. There exist several ATR-crystals; ZnSe, ZnS, Ge, Si and diamond. The principle of ATR is that the IR beam enters the crystal in an angle of 45°. The IR-beam enters into the sample and this part of the light is called the evanescent wave. The different crystals have different refractive index and this index affect the depth of penetration, seen from equation 8. The sample will absorb some of the energy and this wave will be attenuated. The exiting light is then detected by the detector. The different crystals have different penetration depths, ranging from 0.65 to 1.66 µm at an angle of 45° and intensity of 1000 cm-1, where the germanium crystal has the shortest penetration. The germanium crystal also has the highest refractive index which makes it a good crystal to analyze highly absorbing samples [27].

Equation 8, used from calculating the penetration depth of the IR light involves, λ wavelength, α incidence angle of light, n1 and n2 the refractive index of the crystal and sample respectively and θ incidence angle [29]. From the equation, it is evident that the depth of penetration will change depending on the wavelength of the light. This means that when a sample is analyzed within a wavelength range, for example between 4000-600 cm-1 the longer wavelengths will penetrate deeper into the sample than the shorter wavelengths with more energy. The result is that the longer wavelengths will penetrate deeper into the material, hence analyze the bulk properties. Meanwhile, the shorter wavelengths will analyze the surface properties.

𝒅𝒑=

𝝀

𝟐𝝅𝒏𝟏(𝒔𝒊𝒏𝟐𝜶−𝒏𝟐𝟏𝟐 )𝟏/𝟐 (8)

7.8.2 Agilent 4300 Handheld FTIR Spectrometer

The handheld FTIR spectrometer is an equipment that permits easier measurements since it can be used in field, where the material is situated, without sample collection and sample preparations. To the Agilent 4300 FTIR spectrometer, five interchangeable interfaces can used to fit the purpose. The interfaces available are a diamond ATR sensor that analyzes to a depth of 2-3 microns from the surface. Further there is a diffuse reflectance (when low reflectance of light), external reflectance (films on metal substrates, when light is reflected of the surface) and a grazing angle interface (for thin films and higher incidence angle). Lastly, there is a germanium ATR that detects a thinner part of the surface than the diamond ATR, 0.5-2 microns. This is considered as the appropriate interface for analyzing samples with high absorption [30]. Another handheld FTIR equipment have been tested by the company that produces the equipment, namely the Agilent 4100 ExoScan FTIR, onto O-rings, seals and gaskets containing carbon black [31]. A germanium ATR interface, Ge ATR, was used due to the carbon black. It was seen that the Ge ATR gave clearer spectrum than the one from the diamond ATR interface [31].

7.9 Micro Chamber analysis

(16)

12

8 Motivation to choice of material and aging environment

This is the first time Element Materials Technology uses the handheld FTIR equipment onto rubbers and the main goal is to see the potentials with this analysis method. Since this is the beginning of the research, it seemed important to make the test program as wide as possible. By this, it means that no specific case was of interest but rather there was of interest to investigate whether the handheld FTIR could be used for additional types of materials and investigations than the ones already established. Bearing this in mind, some choices were made regarding materials and aging environment. Three types of rubbers were used; NBR and two NR rubbers, and they were all thermally aged. The reason behind the choice of NBR is its great use in the aerospace industry. NR was chosen since it is not considered as aging resistant as NBR. Two different NR were used, which had different hardness. By choosing both an aging and a not aging resistant material, it might be possible to see if the FTIR method has any limitations. Furthermore, the NBR rubber was black and the two NR rubbers used were beige and white respectively. This can also be an interesting perspective since problems may arise when analyzing high absorbing materials (due to carbon black content) with FTIR.

The rubbers were thermally aged in 50°C and 70°C. A temperature of 70°C is a common test temperature for rubber tests according to many previously made researches and since it is a common test temperature at Element Materials Technology. By also choosing a lower temperature it could hopefully be possible to see if there are any limitations using the FTIR technique. The choice of looking at thermally aged rubbers was because heat is a common aging environment and further it is easier to perform short experiments do develop a suitable FTIR analysis method before analyzing the samples from the long term aging experiment.

9 Materials, methods and instruments

9.1 Materials

The material used was one nitrile butadiene rubber and two types of natural rubbers, one harder and one softer, see table 1. The rubbers were bought from a company which is only a supplier and not producers of rubbers. The information about the actual composition of the rubbers and the nitrile content of the nitrile butadiene rubber could not be received from the supplier.

Table 1: Used rubbers during the project. There is one NBR rubber and two NR rubbers, one softer and one harder, which are named accordingly. The hardness stated is received from data sheets from the supplier.

Rubber Abbreviation Appearance Hardness (°Shore A)

Nitrile butadiene rubber NBR Black 65 ± 5

Natural rubber NR_1 Beige 40 ± 5

Natural rubber NR_2 White 68 ± 5

9.2 Methods

9.2.1 Pre-study and developing of an analysis method

In the pre-study it was of interest to develop methods for analysis with both the stationary and the handheld FTIR equipment. Therefore, short term, thermal aging experiments of the rubbers were performed in various temperatures and thereafter analyzed with the stationary FTIR.

When methods for analysis with the stationary FTIR were developed, an investigation about the potentials by using the handheld FTIR was performed and an analysis method was developed. Methods developed from the pre-study were then applied for the rubbers subjected to a long term, thermal aging.

(17)

13 9.2.2 Long term aging

A long term thermal aging of the rubber samples in oven was performed to investigate the changes of tensile properties, hardness, compression set and changes in mass and volume. Further, samples were also aged and analyzed with TGA and FTIR spectroscopy.

9.2.3 Thermal gravimetric analysis, TGA

TGA analysis was performed to investigate the composition of the rubbers; volatiles, polymer, combustible matter and residues. It was also of interest to investigate how the thermal aging affected the different components. Further, it was also of interest to see if the thermal stability changed after the thermal aging.

9.2.4 Fourier Transform Infrared Spectroscopy, FTIR, stationary

FTIR spectroscopy was used to analyze and characterize the changes in the rubber and to see migrating additives. Both an UATR and a Slide Holder accessory were used to the stationary FTIR.

9.2.5 Fourier Transform Infrared Spectroscopy, FTIR, handheld

The handheld FTIR analysis method was as well as the stationary technique used to characterize the changes of the rubber. The results from the handheld FTIR analysis were compared to the results from the stationary FTIR. Four different interfaces were tested together with the handheld equipment, see section 9.3.1.

9.2.6 Micro Chamber

A Micro Chamber technique was used to analyze the volatile organic compounds, VOCs that were leaving the rubbers during heating. This analysis was performed by an operator at Arbets- och miljömedicinska laboratoriet in Linköping.

9.3 Instruments

Table 2 shows the instruments used during the experiment.

Table 2: Specifications regarding used equipment and respective UN numbers.

Apparatus UN-number Specifications

TGA UN-2520 TGA TA Instruments Q500

FTIR Stationary  UATR  Slide Holder

UN-2737 Perkin Elmer Spotlight 200/Frontier

FTIR Handheld

 ATR

 Diffuse reflectance  External reflectance  Grazing angle

- Agilent 4300 Handheld FTIR

Cabinet oven UN-1643, UN-1659 -

Punch-machine UN-1698 -

Durometer (°Shore A) UN-3001 -

Compression set Thickness gauge - UN-3068 - -

Tensile testing machine Extensometer

UN-1561 UN-2873

- -

Density measurments UN-3022 -

9.3.1 Handheld FTIR

(18)

14

Table 3: The four different interfaces and the abbreviation used during the experiments. Four different backgrounds were used, abbreviated according to the table. All the backgrounds were tested for every interface, however, only the ones stated in the list were possible to use for the respective interface.

Interface Abbreviation of interface

Background Abbreviation of background

ATR A Air

Diffuse reflectance B Coarse silver s

External reflectance C Coarse gold g

Grazing Angle D Mirror gold mg

10 Experimental

10.1 Thermal aging

Thermal aging in oven was performed in several stages in the project. In the beginning of the project, a long term aging was started. Thereafter, shorter aging experiments were performed with intent to develop suitable analysis methods with the stationary and handheld FTIR equipment, meanwhile the long term aging was running. In the early stage of the project it was of interest to investigate whether migration of additives could be observed with both the stationary and handheld FTIR equipment. Therefore, shorter thermal aging experiments were done in several different temperatures. Further, in some experiments, different preparations were done on the rubbers before the thermal aging.

10.1.1 Short term aging – pre-study

The aim was to see if it was possible to get any migration of additives to the rubber surface. It was also of interest to investigate if commonly used solvents could be used to dissolve/collect additives that had migrated to the rubber surface. Further, it was also of interest to analyze if the solvents affected the surface properties. These effects were analyzed with FTIR. The two solvents used were solvent 1 and 2, see table 4.

Table 4: Solvents used for cleaning the rubber surfaces and for analysis with Slide Holder FTIR analysis. Solvent number Abbreviation of solvent Content

1 S1

Ethanol Methyl isobutyl ketone

Methyl ethyl ketone

2 S2

Ethyl acetate Ethanol

Rubber samples were aged in 50°C, 70°C and 90°C between 18-24 hours to investigate if there were substances on the surface that could be wiped off with the two different solvents. These rubbers were not analyzed with FTIR but rather visually inspected by wiping the rubber surface with the two different solvents. The rubber surfaces were cleaned with either of the solvents 1 or 2 before put in oven. The cleaning of the rubber surface was made by wiping the surface with a paper cloth with the solvent on it.

(19)

15

Table 5: The table summarizes what solvent that was used to clean the rubber surface. Some of the samples were not cleaned. Further, only some of the samples were aged. In the case that the rubber surface was cleaned before aging, the same solvent was used to analyze with Slide Holder-FTIR. The same procedure was made for all the three rubber samples NBR, NR_1 and NR_2.

Rubber sample Solvent used for cleaning surface

(S1 or S2)

Temperature for thermal aging

(°C)

Solvent for analysis with Slide Holder FTIR

(S1 or S2)

1 x x S1

2 S1 90 S1

3 x x S2

4 S2 90 S2

Table 6: The table summarizes what solvent that was used to clean the rubber surface. Some of the samples were not cleaned. Further, only some of the samples were aged. The rubber surfaces were analyzed with FTIR-UATR. The same procedure was made for all the three rubber samples NBR, NR_1 and NR_2.

Rubber sample Solvent used for cleaning surface before aging

(S1 or S2)

Temperature for thermal aging (°C) 1 x x 2 x 90 3 S1 x 4 S1 90 5 S2 x 6 S2 90

10.1.2 Long term aging

The long term aging was performed in a cabinet oven at temperatures of 50°C and 70°C. The test pieces was dumb bell shapes, test pieces for compression set and hardness measurements and square shapes of dimensions 40 mm x 40 mm with a thickness of 2 mm used for FTIR analysis. Also a test piece was prepared for TGA analysis, with a thickness of 2 mm. The long term aging was performed during six weeks which according to the experience Element Materials Technology received from previously made micro calorimetric analysis made together with other partners, corresponds to 11.5 months (T=50°C) and 3 years and 10 months (T=70°C). The reference temperature used for calculating the number of years was selected to 20°C, since this temperature is usually used by Element Materials Technology. With the assumption that the reaction speed is doubled for an increase of the temperature by 10°C, one day in 50°C corresponds to 8 days in 20°C and one day in 70°C corresponds to 32 days in 20°C.

The samples were taken out from the oven and were left for at least 16 hours before any analysis were started, according to ISO 188.

10.2 Fourier Transform Infrared Spectroscopy, stationary

(20)

16

solvent was evaporated before analysis in the Slide Holder accessory. A clean crystal was used as a background. Number of scans was four and the scanning range was 4000-450 cm-1. The NaCl crystal was used when solvent 1 was used as a solvent and the ZnSe was used for solvent 2.

10.3 Fourier Transform Infrared Spectroscopy, handheld

The handheld FTIR measurements were performed with Agilent 4300 Handheld FTIR equipment. Four different interfaces were used; ATR, diffuse reflectance, external reflectance and a grazing angle interface and were named according to table 3 in section 9.3. Different backgrounds were used for the different interfaces, see table 3. The measurements were made by pressing the interface towards the surface of the rubbers. Different parameters were varied in the instrument settings for the different interfaces. Analysis-methods were created, numbered 1-13, see table 8. The different methods tested and the respective result can be found in table 8, section 11.2.

10.4 Mechanical testing

10.4.1 Hardness measurements

The hardness was measured on the materials, according to standard ASTM D2240-15. The hardness of the samples was taken at a standard laboratory environment. Information about the durometer can be seen in table 2. Three pieces was plied to achieve a total thickness of at least 6 mm, were each test piece was at least 2 mm thick. The hardness of the material was calculated as a median of five measurements of each piece. For every test piece the change in hardness before aging and after were calculated and the results stated are the median change of the hardness of three test pieces for each material.

10.4.2 Compression set

ISO 815:1991(E) was followed to measure the compression set after the test pieces had been subjected to compression for 24h in 100°C. The test pieces used had a diameter of 29 mm whereby it falls under the test piece of type A according to ISO 815. However, the thickness deviates from this test piece type, where the thickness of the NBR samples and NR_1 had a thickness of 6 mm and NR_2 had a thickness of 2 mm. Neither of the test pieces was laminated. Three individual test pieces was used for each material and the median was calculated. Since the purpose of the test was to see the change before and after aging, the deviation from a standard test piece does not seems to influence the result since the same test method is used for both an unaged and an aged sample. Unaged reference samples were used to calculate the percentage change between aged and non-aged specimens.

10.4.3 Tensile measurements

The tensile measurements were performed according to ISO 37:2017(E). A type 2 dumb-bell was used and chosen due to its small shape. The thickness was measured in the narrow part of the dumb-bell and the reported value is the median taken of three measurements. The median out of three measurements were then used to calculate the change of the property before and after aging and the reported values are the median change, see table 11. The test pieces were conditioned in a standard test environment before testing and the temperature of testing was 23°C and a relative humidity of 50 %. A number of 3 individual test pieces were tested. The test speed was 500 mm/min.

10.4.4 Change in weight and volume

The change in weight and volume was performed according to ISO 1817:2015(E). The materials were conditioned in standard laboratory environment before testing. The same procedure was followed when testing the aged samples. The results are expressed as medians of three measurements.

10.5 Thermogravimetric analysis, TGA

(21)

17

ASTM E1131-08 (2014) with the exception of the temperature ramping of 5°C/min, that was considered as more appropriate for the purpose of this analysis. The results was analyzed according to the procedure where the highly volatiles are classified into the region of 20-150°C, volatile matter 150-250°C, medium volatile 250-580°C, combustible between 580-800°C and residue from 800-900°C.

10.6 Micro Chamber

The Micro Chamber analyses were performed by an operator at Arbets- och miljömedicinska laboratoriet in Linköping, on NBR and NR_2. Samples were placed in the Micro-Chamber and analyzed for 10 minutes in 30°C, 40°C and 50°C. The volatile organic compounds were detected and analyzed by GC/MS. The five compounds with the highest abundance in the samples were compared to each other.

11 Results and discussion

11.1 Short term aging – pre-study

11.1.1 Aging in 50°C and 70°C for 72 h – UATR-FTIR

(22)

18

Figure 4: Spectrums for sample NR_1. The top two spectrums show unaged samples, middle two aged samples aged in 50°C and the two lowest aged in 70°C. The aged samples were aged 72 hours.

(23)

19

Figure 6: Spectrums for sample NBR. The top spectrum show unaged sample, middle two aged samples in 50°C and the two lowest aged in 70°C. The aged samples were aged 72 hours.

11.1.2 Aging in 90°C for 12 days – Slide Holder FTIR

The analysis with the Slide Holder-FTIR was performed on rubbers that had been aged for a shorter time period. The rubbers were aged at a temperature of 90°C to hopefully induce migration of additives to the rubber surface. It was of interest to see if this method could collect additives and analyze them by help of the stationary FTIR equipment. Two solvents were chosen based on their common usage and since they are used for cleaning of rubber surfaces at Element Materials Technology. The analysis was started with analyzing if solvent 1 and 2 could be used together with the lens paper to ensure that they did not affect the paper. It could be seen from the spectrums that this was not the case, figure 45 in appendix A.

(24)

20

Figure 7: The top two spectrums correspond to unaged NBR samples and the two lower ones are NBR samples that had been aged in 90°C for 12 days. The spectrums show what was collected with solvent 1 and 2 respectively from respective rubber.

(25)

21

Figure 8: The top two spectrums correspond to unaged NR_1 samples and the two lower ones are NR_1 samples that had been aged in 90°C for 12 days. The spectrums show what was collected with solvent 1 and 2 respectively from respective rubber.

11.1.3 Aging in 90°C for 6 days – UATR-FTIR

(26)

22

Figure 9: The effect of the solvents on the NBR rubber surface were analyzed by comparing samples that were unaged and were cleaned and not cleaned respectively.

(27)

23

several different bonds, possibly an alkyl halide or a carboxylic bond. The peak below 1500 cm-1 can correspond to CH2, CH3 or S=O bond.

Figure 11: The figure show NR_1 samples. The top sample is an unaged sample. The two following are unaged and cleaned with solvent 1 and 2 respectively. The forth samples from the top is aged in 90°C without any pre-treatment. The two lowest spectrums are aged in 90°C and cleaned with solvent 1 and 2 respectively before thermal aging.

(28)

24

Figure 12: The figure show NR_1 samples. The top sample is an unaged sample. The two following are unaged and cleaned with solvent 1 and 2 respectively. The forth samples from the top is aged in 90°C without any pre-treatment. The two lowest spectrums are aged in 90°C and cleaned with solvent 1 and 2 respectively before thermal aging.

11.1.4 Aging in 90°C for 6 days – Slide Holder FTIR

(29)

25

Figure 13: The figure show NBR samples. The top sample is an unaged sample. The two following are unaged and cleaned with solvent 1 and two respectively. Forth from the top is aged in 90°C without any pre-treatment. The fifth and the sixed spectrums are aged in 90°C and cleaned with solvent 1 and 2 respectively before thermal aging. The two lowest spectrums are analyzed with the Slide Holder technique.

(30)

26

Figure 14: The figure show NR_1 samples. The top sample is an unaged sample. The two following are unaged and cleaned with solvent 1 and two respectively. Forth from the top is aged in 90°C without any pre-treatment. The fifth and the sixed spectrums are aged in 90°C and cleaned with solvent 1 and 2 respectively before thermal aging. The two lowest spectrums are analyzed with the Slide Holder technique. Analysis of the rubber surfaces. 11.1.5 Analysis of the rubber surfaces

The rubber surfaces were visually analyzed as described in section 10.1.1. The results from this analysis, see table 7, strengthen the results seen for NR_2 in section 11.1.2. There was no sign of any migrating additives to the surface in the FTIR spectrum. As can be seen in table 7, there was neither any visual sign of any migrating additives when the NR_2 surface was wiped. However, the samples that had been aged shorter, a peak at 3000 cm-1 could be seen in the spectrum, figure 44 appendix A.

(31)

27

Table 7: Results from cleaning before and after aging in oven or on reference samples. The samples that were “surface roughened” were roughened with a sand paper.

Cleaning and aging procedure

Result on NBR Result on NR_1 Result on NR_2

Solvent 1 on unaged samples No yellow No yellow No yellow

Solvent 2 on unaged samples Yellow Yellow No yellow

Solvent 1, aging (50°C,70°C, 90°C), solvent 2

Yellow Yellow Nothing

Solvent 1-surface roughing- solvent 1- 90°C oven aging-

solvent 1

No yellow A bit yellow No yellow

Solvent 1-surface roughing- solvent 1- 90°C oven aging-

solvent 2

A bit yellow A bit yellow No yellow

11.2 Pre-study - Fourier Transform Infrared Spectroscopy, handheld

In the software to the handheld FTIR equipment there are pre-installed methods created for the different interfaces. The analysis with the handheld FTIR started with analyzing how the spectrums became when using the pre-installed methods, thereafter changing three parameters in the methods. The different methods created and tested can be seen in table 8. The three different parameters that were changed were the number of background scans, the resolution and the number of sample scans. Element Material Technology has not tested the handheld FTIR onto rubbers before, why much time was spent on analyzing the effects of developing the analysis methods. It was of interest to see if an appropriate analysis-method could be developed.

Table 8: Method A-, B-, C-, and D-, refers to the interfaces stated in table 3. The effects of changing three parameters were investigated: the number of background scans, the resolution and the sample scans.

Method (A-, B-,C-,D-) Background scans (no.) Resolution (cm-1) Sample scans (no.) 1 32 4 32 2 32 8 32 3 32 16 32 4 32 2 32 5 64 8 64 6 128 8 128 7 256 8 256 8 128 8 64 9 256 8 128 10 256 4 128 11 64 8 32 12 64 16 32 13 64 4 32

11.2.1 ATR interface- NR_1 – developing an analysis method

The ATR interface was first tested onto NR_1. The reason to the choice of NR_1 was the fact that with the stationary FTIR, a change in the UATR spectrum could be observed between unaged and aged samples. Therefore, it seemed best to choose this rubber to create a good analysis method. The number of background scans should always be higher or the same as the number of samples scans. Figure 15 to 18 shows how the spectrum changed when changing in between the methods presented in table 8.

(32)

28

Figure 15: NR_1 sample, analyzed with method A1, A2 and A3 (ATR interface).

Figure 16 shows that doubling the number of background scans while keeping the resolution and the sample scans constant increased the absorbance. However, increasing the number of background and sample scans (same number of scans for both background and sample) while keeping the resolution constant (8 cm-1) did not changed the spectrums, see figure 55 in appendix B.

Figure 16: NR_1 sample, analyzed with method A5 and A8 (ATR interface).

(33)

29

Increasing or decreasing the resolution from 8 cm-1 to 16 cm-1 or 4 cm-1, at fixed background and samples scans of 64 and 32 respectively, did not became better than the resolution of 8 cm-1, figure 18. Figure 18 also show that the highest absorbance was achieved when using method A11.

From the performed analysis, it was decided to continue with method A11 onto the aged rubbers. These rubbers had previously been analyzed with the stationary FTIR why it was interesting to see if the same things could be observed with the handheld equipment. Some of the other methods were also tested onto unaged NBR and NR_2; however method A11 seemed like the most appropriate method for further analysis.

11.2.2 ATR interface- aged samples in 90°C for 6 days

(34)

30

Table 9: Naming of aged samples analyzed with the handheld FTIR equipment. Both NR_2 and NBR were named in the same way as NR_1. A11 stands for the method and xx is the measurement number (01-03).

Naming of samples Sample description NR_1 A1101 Reference sample, method A11

NR_1 A11_90_03 Reference sample, method A11, aged in 90°C

NR_1_A11_S1_01 Reference sample, method A11, cleaned with solvent 1

NR_1_A11_90_S1_02 Method A11, aged in 90°C, cleaned with solvent 1 before aging

NR_1_A11_S2_03 Reference sample, method A11, cleaned with solvent 2

NR_1_A11_90_S2_01 Method A11, aged in 90°C, cleaned with solvent 2 before aging

Figure 19: NR_1 samples, for both unaged and aged samples. The naming and pre-treatment of the samples can be seen in table 9.

(35)

31

Figure 21: NBR samples, for both unaged and aged samples. The naming and pre-treatment of the samples can be seen in table 9.

(36)

32 11.2.3 Diffuse reflectance interface

Several methods were tested for NR_1 with the diffuse reflectance interface; see additional figures in appendix B, figure 59-61. However, figure 23 shows that the spectrums contained more noise and did not had the same appearance as the spectrums from ATR. Since the spectrums did not show the same quality it was decided not to perform the analysis onto aged samples. Especially, the spectrums from NBR had a lot of noise, figure 64 and 65 appendix B. Also the spectrum from NR_2 can be found in appendix B, figure 62 and 63.

Figure 23: Sample NR_1 analyzed with method B11, B8 and B9 (Diffuse reflectance interface). The “s” stands for the background “coarse silver”.

11.2.4 External reflectance interface

Comparably to the diffuse reflectance, the external reflectance gave lower noise and a clearer spectrum for both NR_1 and NBR, figure 24 and 25 respectively. The spectrum from NR_2 can be found in appendix B, figure 66.

(37)

33

Figure 25: Sample NBR analyzed with method C10, C11, C12 and C13 (External reflectance interface). The “g” stands for the background “coarse gold”.

11.2.5 Grazing angle interface

The grazing interface gave rise to the spectrums seen in figure 26 for NR_1. Additional spectrums for NBR and NR_2 can be found in appendix B, figure 67 and 68.

Figure 26: Sample NR_1 analyzed with method D10, D11, D12 and D13 (Grazing angle interface). The “mg” stands for the background “mirror gold”.

11.2.6 Comments on developing a method for the handheld FTIR

Investigating the Possibilities of Using a Handheld-FTIR Equipment to Characterize Thermal Aging of Rubbers

Investigating the Possibilities of

Using a Handheld-FTIR Equipment

to Characterize Thermal Aging of

Rubbers

Under supervision of Simon Pettersson, Per Reinholdsson

and Mikael Hedenqvist

ANTONIA TENGBOM

1

Abstract

2

Sammanfattning

3

Acknowledgements

4

Contents

5

Introduction

6

Objectives

7

Literature research

𝒌 = 𝑨 ∗ 𝒆

8

Motivation to choice of material and aging environment

9

Materials, methods and instruments

10

Experimental

11

Results and discussion