(Melamine Phosphate) on the Thermal Properties and Flame ... - MDPI

materials Article

Effect of POSS Particles and Synergism Action of POSS and Poly-(Melamine Phosphate) on the Thermal Properties and Flame Retardance of Silicone Rubber Composites 1, * Przemysław Rybinski ´ 1 2

*

ID

2 ˙ , Bartłomiej Syrek 1 , Dariusz Bradło 2 and Witold Zukowski

´ etokrzyska 15, Faculty of Mathematics and Natural Science, Jan Kochanowski University, Swi˛ Kielce 25-406, Poland; [email protected] Faculty of Chemical Engineering and Technology, Cracow University of Technology, Warszawska 24, ˙ Cracow 31-155, Poland; [email protected] (D.B.); [email protected] (W.Z.) Correspondence: [email protected]

Received: 25 June 2018; Accepted: 24 July 2018; Published: 27 July 2018







Abstract: This article presents flame retardant compounds for silicone rubber (SR) in the form of polyhedral oligomeric silsequioxanes (POSS), containing both isobutyl groups and amino-propyl (AM-POSS) or chloro-propyl group (HA-POSS) or vinyl groups (OL-POSS). Silsequioxanes were incorporated into the silicone rubber matrix in a quantity of 3 and 6 parts by wt by the method of reactive stirring with the use of a laboratory mixing mill. Based on the analyses performed by TG (Thermogravimetry) FTIR (Fourier Transform Infrared Spectroscopy), conical calorimeter, and SEM-EDX (Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy) methods, the thermal degradation mechanism of non-cross-linked and cross-linked silicone rubber has been elucidated. The effects of POSS, and POSS in a synergic system with melamine polyphosphate (MPP), on the thermal properties and flammability of silicone rubber composites were presented. Based on the test results obtained, a mechanism of flame retardant action POSS and POSS-MPP has been proposed. It has been shown that POSS, especially with MPP, considerably increases the thermal stability and decreases the flammability of the SR rubber composites under investigation. Keywords: silicone rubber; polyhedral oligomeric silsesquioxanes (POSS); melamine polyphosphate; thermal properties; flammability

1. Introduction Silicone rubbers (SR) are characterized by excellent compatibility, and chemical, thermal, and UV resistances, and also showing very good dielectric properties. Unfortunately, silicone rubbers are flammable polymers; when ignited they are totally burnt, releasing great amounts of heat within a short period of time. The flammability of silicone rubbers considerably reduces their use in the medical, automotive, power, and cable industries [1,2]. A commonly used method of improving the resistance of polymeric materials, including silicone rubbers, against the action of fire consists of incorporating additive flame-resistant compounds into their matrix. On account of environmental restrictions concerning the use of halogen flame retardants, at present halogen-free organophosphorus flame-retardant compounds, such as melamine polyphosphate (MPP) or ammonium polyphosphate (APP), have the greatest application potential [3]. The action of organophosphorus compounds consists in forming an expanded, intumescent carbon layer on the polymer surface during its thermal decomposition. This layer acts as a barrier preventing heat transfer from its source to the polymer surface and reducing the fuel transfer to the flame [4]. Materials 2018, 11, 1298; doi:10.3390/ma11081298

www.mdpi.com/journal/materials

Materials 2018, 11, 1298

2 of 19

To obtain a high efficiency of flame retardant compounds, one has to incorporate them into the polymeric matrix in the presence of carbon-rich compounds. During the thermal decomposition of a polymeric composite, the organophosphorus compounds contained in it, e.g., MPP, undergo degradation to water, ammonia, and PP acid that when reacting with carbon compounds, forms thermally unstable esters of phosphoric acid. Their decomposition, mainly by dehydration processes, results in the formation of a carbon layer cutting off the flow of oxygen, fuel, and heat between the polymer and flame. Water vapor and nitrogen compounds formed during the MPP decomposition diffusing to the flame, foam the liquid destruction products of the composite and increase the insulating power of the boundary layer [5]. The incorporation of MPP into the poor in carbon silicone rubber to make it flame retardant must be combined with a carbonizing additive, such as pentaerythritol (PER) [6]. From the investigations performed so far, it distinctly follows that the satisfactory effectiveness of the MPP/PER system, especially in relation to silicone rubber, can be obtained when its content is over 30 parts by wt. Unfortunately, such a high quantity of this flame retardant negatively affects the processing, functional properties, and price of the silicone rubber composites obtained [7]. New, alternative of halogen and halogen-free flame-retardant compounds are polyhedral oligomeric silsequioxanes (POSS) [8,9]. POSS have the structure of cage whose edges are built of combined silicone and oxygen atoms. Organic ligands are directly linked to silicon atoms, owing to which, the cage can be combined with polymer chains by means of covalent bonds, forming hybrid composites. The size of silicate cages ranges from 1 to 3 nm, hence they are treated as the smallest silica particles to be obtained [10]. POSS can be incorporated as reactive components during polymerization or combined with macromolecules by their direct plasticization with the polymeric matrix. From the investigations performed so far, it follows that POSS incorporated into silicone rubbers considerably improves their mechanical and thermal properties and reduces their flammability [11]. Liu et al. [12] have found that POSS with various functional groups, such as phenyl-trisilanol, isobutylo-trisilanol, and isobutyl-monosilanol groups, increases the thermal stability of silicone rubber nanocomposites. As a result of the tests performed, it has been shown that room temperature vulcanized silicone rubber (RTV-SR), containing 5% of tri-silanol-isobutyl POSS, is characterized by the highest thermal stability among the composites tested. Other studies have shown a synergic effect of silica and POSS on the improvement in thermal and mechanical properties of silicone rubber (RTV-SR) [12]. The use of POSS as effective compounds for reducing the flammability of polymers is presented in a review by Zhang and co-workers [9]. From the review of the source literature, it follows that in the past, attempts were undertaken to examine and explain the mechanism of the synergic action of the POSS-phosphorus compounds on the flammability of polypropylene, polylactide, polyester, and epoxide resins or butyl polysuccinate [11,13–18]. Carosio et al. [19] studied the effect of layer by layer assemblies on the thermal and combustion behavior of acrylic fabrics. Bi-layers consisting of APP/POSS pairs have been deposited on acrylic fibers, creating a homogeneous and regular coating able to completely cover the fibers. Researchers indicated that the thermal degradation in an inert atmosphere, as well as the thermo-oxidation in air of the acrylic fibers, has been significantly modified by the coatings due to their char-former character [19]. Despite the investigations performed, the effect of POSS, especially the POSS-phosphorus compounds system, on the flammability of polymeric materials has not been totally explained. The present paper describes pioneering studies on the effect of three POSS compounds containing amino groups (AM-POSS), chlorine groups (HA-POSS), and vinyl groups (OL-POSS) on the thermal properties and flammability of high-temperature vulcanized silicone rubber (SR), also taking into account the decomposition kinetics. The effect of the synergism of POSS-MPP action on the flammability of silicone rubber was also examined. Based on the analyses performed by TG, FTIR,

2.1. Materials The object of study was methylvinylsilicone rubber (SR), made by “Silikony Polskie” (Nowa, Sarzyna, molecular weight of 60–70 × 104 (kg/mol) andbyvinyl groupPolskie” content of 0.05– The Poland), object of with studya was methylvinylsilicone rubber (SR), made “Silikony (Nowa, 0.09%. The rubber was cross-linked with the use of bis(2,4-dichlorobenzoil) peroxide from “Silikony Sarzyna, Poland), with a molecular weight of 60–70 × 104 (kg/mol) and vinyl group content of 0.05– Polskie” in rubber a quantity 0.7 part bywith wt/100 parts wt. of the rubber (Vulcanised rubber 0.09%. The was of cross-linked the use ofby bis(2,4-dichlorobenzoil) peroxide silicone from “Silikony signed as SR OP, where OP is organic perioxide). Materials 2018, 11, 1298 3 of 19 Polskie” in a quantity of 0.7 part by wt/100 parts by wt. of the rubber (Vulcanised silicone rubber The following compounds were used as fillers for the rubber blends: melamine polyphosphate signed as SR OP, where OP is organic perioxide). MPPThe from Everkemcompounds (Milano, Italy) chemical of MPPmelamine is C3H6N6polyphosphate (H3PO4)n, and following were(Scheme used as 1), fillers for theformula rubber blends: conical calorimeter and silsesquioxanes SEM-EDX methods, thefrom mechanism of the flame-retardant action of POSS-MPP polyhedral oligomeric POSS Hybridplastics (Hattiesburg, USA). The chemical MPP from Everkem (Milano, Italy) (Scheme 1), chemical formula of MPP is C3H6N6 (H 3PO4)n, and was explained. formula of POSS is (RSiO 1.5)n where R is a hydrogen atom or an organic group (Table 1). Please give polyhedral oligomeric silsesquioxanes POSS from Hybridplastics (Hattiesburg, USA). The chemical the manufacture’s location. formula of POSS is (RSiO1.5)n where R is a hydrogen atom or an organic group (Table 1). Please give 2. Experimental the manufacture’s location. 2.1. Materials The object of study was methylvinylsilicone rubber (SR), made by “Silikony Polskie” (Nowa, Sarzyna, Poland), with a molecular weight of 60–70 × 104 (kg/mol) and vinyl group content of 0.05–0.09%. The rubber was cross-linked with the use of bis(2,4-dichlorobenzoil) peroxide from “Silikony Polskie” in a quantity of 0.7 part by wt/100 parts by wt. of the rubber (Vulcanised silicone rubber signed as SR OP, where OP is organic perioxide). The following compounds were used as fillers for the rubber blends: melamine polyphosphate MPP from Everkem (Milano, Italy) (Scheme 1), chemical formula of MPP is C3 H6 N6 (H3 PO4 )n , and polyhedral oligomeric silsesquioxanes POSS from Hybridplastics (Hattiesburg, USA). The chemical Scheme 1. Melamine polyphosphate structure. formula of POSS is (RSiO1.5 )n where R is a hydrogen atom or an organic group (Table 1). Please give the manufacture’s location. Scheme 1. Melamine polyphosphate structure. Table 1. Chemical structure of POSS nanoparticles.

Table of nanoparticles. Table 1. 1. Chemical Chemical structure structure of POSS POSS nanoparticles. Material Chemical Structure

Material

Material

Chemical Structure Chemical Structure

AM-POSS AM-POSS AM-POSS

HA-POSS HA-POSS HA-POSS Materials 2018, 11, x FOR PEER REVIEW

4 of 19

OL-POSS OL-POSS

2.2. Methods Preparation of Rubber Blends Rubber blends were prepared using a laboratory two-roll mill, with roll dimensions of D = 140 mm and L = 300 mm. The average temperature of the rolls was 35 °C. The rotational speed of the front roll was Vp = 20 rpm, friction 1:1. The composition of the rubber mixtures are shown in Table 2. Table 2. Composition of the silicone (SR) rubber-based mixtures.

signed as SR OP, where OP is organic perioxide). The following compounds were used as fillers for the rubber blends: melamine polyphosphate MPP from Everkem (Milano, Italy) (Scheme 1), chemical formula of MPP is C3H6N6 (H3PO4)n, and polyhedral oligomeric silsesquioxanes POSS from Hybridplastics (Hattiesburg, USA). The chemical Materials 11, 1298is (RSiO1.5)n where R is a hydrogen atom or an organic group (Table 1). Please4 give of 19 formula2018, of POSS the manufacture’s location.

Scheme 1. Melamine polyphosphate structure. Scheme 1. Melamine polyphosphate structure.

2.2. Methods

Table 1. Chemical structure of POSS nanoparticles.

Preparation of Rubber Blends Material

Chemical Structure

Rubber blends were prepared using a laboratory two-roll mill, with roll dimensions of D = 140 mm and L = 300 mm. The average temperature of the rolls was 35 ◦ C. The rotational speed of the front roll was Vp = 20 rpm, friction 1:1. The composition of the rubber mixtures are shown in Table 2.

AM-POSS Table 2. Composition of the silicone (SR) rubber-based mixtures. Component (phr)

Sample Description SR

OP

POSS/AM

POSS/HA

POSS/OL

MPP

R OP 100 SR/AM3 100 SR/HA3 100 SR/OL3 100 SR/AM6 100 SR/HA6 100 SR/OL6 100 SR/MPP HA-POSS 100 SR/AM3/MPP17 100 SR/AM6/MPP14 100 SR/HA3/MPP17 100 SR/HA6/MPP14 100 SR/OL3/MPP17 100 SR/OL6/MPP14 100

0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7

3 − − − 6 − − − 3 6 − − − −

− − 3 − − 6 − − − − 3 6 − −

− − − 3 − − 6 − − − − − 3 6

− − − − − − − 20 17 14 17 14 17 14

SR—silicone rubber; OP—organic peroxide; POSS/AM—Aminopropylisobutyl polyhedral oligomeric silsequioxane; POSS/HA—Chloropropylisobutyl polyhedral oligomeric silsequioxane; POSS/OL—Vinyl polyhedral oligomeric silsequioxane; MPP—melamine polyphosphate.

Kinetic parameters of vulcanization of the elastomer blends were measured by means of MonTech DRPA 300 Rheometer (Buchen, Germany), at 160 ◦ C, according to standard PN-ISO 3417:1994, directly after the preparation of the rubber blends, and for comparison, after a week of conditioning at the ambient temperature. The blends were vulcanized in steel molds placed between electrically heated press shelves. 2.3. Experimental Techniques 2.3.1. FTIR Analysis Fourier transform infrared (FTIR) spectra were recorded on a FTIR Spectrum Two spectrophotometer from PerkinElmer (Waltham, MA, USA). Attenuated total reflection (ATR) accessory equipped with a single reflection diamond ATR crystal on ZeSe plate was used for all of the analysis. Measurement parameters were 32 scans from 500–4000 cm−1 in transmittance mode with a resolution of 8 cm−1 and the speed of scanning equal to 0.6329 cm·s−1 .

Materials 2018, 11, 1298

5 of 19

2.3.2. EDS Techniques EDS analysis of residue after burning were obtained with the use of scanning electron microscope (SEM), model TM3000 from Hitachi (Krefeld, Germany). 2.3.3. Thermal Properties The thermal properties of the rubber composites were tested under an air atmosphere at a temperature ranging from 25 to 700 ◦ C with the use of a thermal analyser (Jupiter STA 449F3, Netzsch Company, Selb, Germany). Weighed portions amounted to about 5–10 mg. Investigated samples were analyzed at a heating rate of 10 ◦ C/min. Determination of activation energy of destruction studied silicone rubber compositions. In order to determine the activation energy of destruction, compositions of silicone rubber were tested under a nitrogen atmosphere at temperatures ranging from 25 to 700 ◦ C with the use of a thermal analyser (Jupiter STA 449F3, Netzsch Company, Selb, Germany). The samples were heated with the rate 5, 10, 15, and 20 ◦ C/min. In general, the thermal degradation reaction of a solid polymer can be simply shown as: Bsolid → Csolid + Dgas, where Bsolid is the starting material, and Dgas and Csolid are the different products during the degradation of Bsolid. In thermogravimetric measurements, the degree of decomposition (conversion) can be calculated as follows [20]: X=

W0 − Wt W0 − W f

(1)

where X is degree of decomposition; Wt , W 0 , Wf are the actual, initial, and final mass of the sample, respectively. A typical model for the kinetic process can be expressed as: dX = k f (X) dt

(2)

where dx/dt is the decomposition rate, f(X), the function of X, which depends on the particular decomposition mechanism, and k is the decomposition rate constant, which can be expressed by the Arrhenius equation:   −E (3) k = A exp RT where A is the pre-exponential factor (s−1 ), E is the activation energy (J/mol), R is the gas constant (8.314 J·mol−1 ·K−1 ), and T is the temperature (K). Substituting Equation (3) into Equation (2), gives   dX −E = A exp f (X) dt RT

(4)

If the temperature of a sample is changed by a constant value of β (β = dT/dt), the variation of the degree of decomposition can be analyzed as a function of temperature. Therefore, the reaction rate gives:   dX A −E = exp f (X) dT β RT

(5)

Equations (4) and (5) are the basic equations for the kinetic calculation. This iso-conversional integral method, suggested independently by Flynn and Wall and Ozawa [21,22], uses Doyle’s approximation of the temperature integral. This method is based on the equations: AE 0.457E log β = log − 2.315 − (6) Rg(α) RT

[21,22], uses Doyle’s approximation of the temperature integral. This method is based on the equations:

Materials 2018, 11, 1298

log β = log

AE 0.457 E − 2.315 − Rg (α ) RT

(6) 6 of 19

where decomposition in kJ/mol; α, the degree of conversion; T, the absolute temperature to reach the conversion, and is thein integral conversion function where decomposition kJ/mol; α, the degree of conversion; T, the absolute temperature to reach the conversion, and is the integral conversion function α

g (α ) =Zα 0

g(α) =

dα fdα (α )

f (α)

(7) (7)

is the integral conversion function Thus, at a constant conversion (α = const.), the plot log β versus (1/T), obtained from a series is the integral conversion function experiments performed at several heating rates, should be a straight line whose slope allows for the Thus, at a constant conversion (α = const.), the plot log β versus (1/T), obtained from a series evaluation of the activation energy: experiments performed at several heating rates, should be a straight line whose slope allows for the evaluation of the activation energy: d (log β ) E slope = = 0.4567 (8)   R  E  dd((1/ log T β)) = 0.4567 (8) slope = 1/T ) rates of 5, R10, 15, and 20 °C/min were chosen. In To apply this iso-conversional method,d(heating 0

this study, the this conversion values of 5, 15, 20, 25, 30, 35,rates 40, 45, 50,10, 55,15, 60,and 65, and have been used, To apply iso-conversional method, heating of 5, 20 ◦ 70% C/min were chosen. which giveconversion α values ofvalues 0.05, of 0.15, 0.25, 0.4,45, 0.45, 0.6,and 0.65, and 0.7 In this would study, the 5, 0.2, 15, 20, 25,0.3, 30,0.35, 35, 40, 50,0.5, 55, 0.55, 60, 65, 70% have respectively for the Flynn-Wall-Ozawa method. been used, which would give α values of 0.05, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, and 0.7 respectively for the Flynn-Wall-Ozawa method. 2.3.4. Flammability 2.3.4.The Flammability SR composites were examined using a cone calorimeter from Fire Testing Technology Ltd. (EastThe Grinstead, UK). Elastomer samples using with dimensions of (100 ×from 100 ±Fire 1) mm and Technology thickness of Ltd. (2 ± SR composites were examined a cone calorimeter Testing −2. During 0.5) mm were tested in a horizontal position with a heat radiant flux density of 35 kW·m (East Grinstead, UK). Elastomer samples with dimensions of (100 × 100 ± 1) mm and thickness of the±tests, the were following were position recorded:with initial sample weight, time to of ignition (2 0.5) mm testedparameters in a horizontal a heat radiant flux density 35 kW(TTI), · m−2 . sample the weight testing,parameters total heat released (THR), initial effective combustion heat to (EHC), average During tests,during the following were recorded: sample weight, time ignition (TTI), weight loss rate (MLR), heat release rate (HRR), and final sample weight. sample weight during testing, total heat released (THR), effective combustion heat (EHC), average weight loss rate (MLR), heat release rate (HRR), and final sample weight. 3. Results 3. Results 3.1. Thermal Properties of Uncured and Cured Silicone Rubber 3.1. Thermal Properties of Uncured and Cured Silicone Rubber The thermal degradation of polysiloxanes (silicone rubbers) can proceed according to three The mechanisms: thermal degradation of mechanism, polysiloxanes (silicone rubbers) can proceed to three reaction unzipping random scission of Si–O bond in according the main chain of reaction unzipping mechanism, random scission of Si–O bond in the main chain of polymer,mechanisms: and external catalytic mechanism. polymer, and external catalytic mechanism. Polydimethylsiloxanes (PDMS), containing silanol (Si–OH) or hydroxyl-alkyl-silanol (Si–R–OH) Polydimethylsiloxanes (PDMS),degradation containing silanol (Si–OH) hydroxyl-alkyl-silanol (Si–R–OH) terminal groups, undergo thermal according to theorunzipping mechanism (Scheme 2) terminal groups, undergo thermal degradation according to the unzipping mechanism (Scheme 2) [20,23,24]. [20,23,24].

Scheme 2. Inter-molecular of hydroxyl-terminated hydroxyl-terminated polysiloxane. polysiloxane. Scheme 2. Inter-molecular mechanism mechanism degradation degradation of

In the first stage of heating, the molecular weight of the polymer violently increases as a result of intermolecular condensation processes proceeding between terminal silanol groups and inter-chain Si–O groups. Further increase in temperature causes a linear decrease in the polymer molecular weight as a result of the formation of cyclic siloxanes followed by their tearing off and passing to the gas phase of cyclic siloxanes, mainly trimers and tetramers. Presumably, the cyclic siloxane that is

Materials 2018, 11, x FOR PEER REVIEW

7 of 19

In the first stage of heating, the molecular weight of the polymer violently increases as a result of intermolecular condensation processes proceeding between terminal silanol groups and interchain Si–O groups. Further increase in temperature causes a linear decrease in the polymer molecular Materials 2018, 11, 1298 7 of 19 weight as a result of the formation of cyclic siloxanes followed by their tearing off and passing to the gas phase of cyclic siloxanes, mainly trimers and tetramers. Presumably, the cyclic siloxane that is stable at at the the polysiloxane polysiloxane degradation degradation temperature temperature can can catalyze catalyze the the resultant resultant degradation degradation processes processes stable of polysiloxane. polysiloxane. of The weakest structure of polydimethylsiloxane (PDMS) is theisC–Si whose whose energy The weakest bond bondininthe the structure of polydimethylsiloxane (PDMS) the bond, C–Si bond, amounts to 326 kJ/mol. Nevertheless, cycliccyclic siloxanes are formed by thebydecomposition of a Si–O energy amounts to 326 kJ/mol. Nevertheless, siloxanes are formed the decomposition of a bond, whose energy value is 451 kJ/mol, which in turn suggests that the thermal decomposition of Si–O bond, whose energy value is 451 kJ/mol, which in turn suggests that the thermal decomposition PDMS should be considered fromfrom the point of view bothofkinetic and molecular structure of PDMS should be considered the point of of view both parameters kinetic parameters and molecular and not only energy. structure and the notbond only the bond energy. Polysiloxanes containing containingterminal terminalgroups, groups,such suchasasa trimethylsilyl a trimethylsilyl vinyl group, as well Polysiloxanes or or vinyl group, as well as as accidently occurring groups the chain, main undergo chain, undergo degradation interaccidently occurring vinylvinyl groups in thein main thermal thermal degradation by inter- by or intraor intra-molecular proceeding between accidental bonds the main polymer chains molecular reactions reactions proceeding between accidental bonds of the mainofpolymer chains (Scheme 3) (Scheme 3) [23,25]. [23,25].

Scheme Scheme 3. 3. Mechanism Mechanismdegradation degradationof ofpolysiloxsane polysiloxsane by by random random scission scission of of Si–O Si–O bond. bond.

Unlike the unzipping mechanism, the thermal degradation of polysiloxanes according to the Unlike the unzipping mechanism, the thermal degradation of polysiloxanes according to the random scission mechanism requires decisively higher temperatures, which causes a violent loss of random scission mechanism requires decisively higher temperatures, which causes a violent loss the sample mass, which is observed already in the early stage of the thermal decomposition of of the sample mass, which is observed already in the early stage of the thermal decomposition of polysiloxane, while the scission of the Si–O bond resulting in the formation of oligomeric final polysiloxane, while the scission of the Si–O bond resulting in the formation of oligomeric final products products generally occurs in the middle part of polysiloxane chain. generally occurs in the middle part of polysiloxane chain. Polysiloxanes containing ionic polar additives or impurities undergo thermal degradation Polysiloxanes containing ionic polar additives or impurities undergo thermal degradation according to a catalytic mechanism. Contrary to the unzipping and random scission mechanisms, the according to a catalytic mechanism. Contrary to the unzipping and random scission mechanisms, catalytic mechanism consists in hydrolytic decomposing the Si–O bond in the main polymer chain. the catalytic mechanism consists in hydrolytic decomposing the Si–O bond in the main polymer chain. Grassie and Macfarlane have shown that even trace quantities of KOH dramatically deteriorate the Grassie and Macfarlane have shown that even trace quantities of KOH dramatically deteriorate the thermal stability of polysiloxanes, even within the range of moderate temperatures [26]. thermal stability of polysiloxanes, even within the range of moderate temperatures [26]. Radhakrishnan [24], investigating the processes of thermal decomposition of polysiloxanes Radhakrishnan [24], investigating the processes of thermal decomposition of polysiloxanes terminated with vinyl group and hydroxyl group, has shown that PDMS terminated with vinyl group terminated with vinyl group and hydroxyl group, has shown that PDMS terminated with vinyl is thermally decomposed according to the random scission mechanism, while in the case of PDMS group is thermally decomposed according to the random scission mechanism, while in the case of containing hydroxyl terminal groups, a mixed mechanism is noticeable, which means that this PDMS containing hydroxyl terminal groups, a mixed mechanism is noticeable, which means that polymer is decomposed with the use of terminal hydroxyl groups, according to the unzipping this polymer is decomposed with the use of terminal hydroxyl groups, according to the unzipping mechanism and the random scission mechanism. mechanism and the random scission mechanism. The methylvinylsilicone rubber tested in the atmosphere of a nitrogen gas underwent thermal The methylvinylsilicone rubber tested in the atmosphere of a nitrogen gas underwent thermal decomposition in a single stage. The maximal rate of this process at a temperature of 483 °C amounted decomposition in a single stage. The maximal rate of this process at a temperature of 483 ◦ C amounted to 22.85%/min (Figures 1 and 2). Under air, this rubber was thermally decomposed in three stages to 22.85%/min (Figures 1 and 2). Under air, this rubber was thermally decomposed in three stages (Figures 1 and 2). The first stage began at T = 350 °C, while the maximal rate of its decomposition at (Figures 1 and 2). The first stage began at T = 350 ◦ C, while the maximal rate of its decomposition at T = 396 °C amounted to 3.80%/min, the second stage began at T = 410 °C, while its maximal rate at T T = 396 ◦ C amounted to 3.80%/min, the second stage began at T = 410 ◦ C, while its maximal rate at = 465 °C◦amounted to 16.90%/min (third stage). T = 465 C amounted to 16.90%/min (third stage).

Materials 2018, 11, 1298 Materials 2018, 11, x FOR PEER REVIEW

8 of 19

Materials 2018, 11, x FOR PEER REVIEW Materials 2018, 11, x FOR PEER REVIEW

8 of 19 8 of 19

8 of 19

Figure TGcurves curvesofofsilicone siliconerubber rubber in in the the air Figure 1.1.TG air and and nitrogen nitrogenatmosphere. atmosphere. Figure 1. TG curves of silicone rubber in the air and nitrogen atmosphere. Figuredecomposition 1. TG curves of silicone rubber in the under air andair nitrogen atmosphere. The three-stage of silicone rubber can result from the catalytic action

decomposition of silicone rubber under air can result from the catalytic action of The airThe inthree-stage the depolymerization reactions of polysiloxane to volatile cyclic low-molecular compounds, three-stage decomposition of silicone rubber under air can result from the catalytic action The three-stage decomposition of silicone rubber under air can result from the catalytic action of air in the depolymerization reactions of polysiloxane to volatile cyclic low-molecular compounds, asair confirmed by a decisively lower temperature of the to beginning of thermal decomposition, TR, of of in the depolymerization reactions of polysiloxane volatile cyclic low-molecular compounds, airrubber in the depolymerization reactions of polysiloxane to volatile cyclic low-molecular compounds, asofconfirmed by a decisively lower temperature of the beginning of thermal decomposition, TR, of SR SR under air in relation to the value of TR under neutral gas. The cyclic compounds, formed as confirmed by a decisively lower temperature of the beginning of thermal decomposition, TR, of asasconfirmed by a decomposition decisively temperature of the neutral beginning ofThe thermal decomposition, TR,then of rubber underunder relation tolower the of TR under cyclic result ofair thein SRvalue rubber thegas. random scission mechanism, formed are SR arubber air in relation toofvalue the of according TR under to neutral gas. The cycliccompounds, compounds, formed as rubber under air in relation to theair value TR under neutral gas. Thescission cyclic formed aSR result of the decomposition SR rubber according the mechanism, then thermally decomposed underof both andof nitrogen at ato temperature 450compounds, °C. as a result of the decomposition of SR rubber according to the random randomabove scission mechanism, areare then as a result of the decomposition of SR rubber according to the random scission mechanism, are then ◦ thermally decomposed temperatureabove above450 450°C.C. thermally decomposedunder underboth bothair airand and nitrogen nitrogen at at aa temperature thermally decomposed under both air and nitrogen at a temperature above 450 °C.

Figure 2. DTG curves of silicone rubber in the air and nitrogen atmosphere. Figure2.2.DTG DTGcurves curves of of silicone silicone rubber Figure rubber in in the theair airand andnitrogen nitrogenatmosphere. atmosphere.

Figure 2. DTG curves of silicone rubber inrubber the air and nitrogen changes atmosphere. The cross-linking of the methylvinylsilicone decisively the character of its thermal under A small weight loss at ΔT decisively between 60changes and 395 the °C ischaracter connectedofwith The decomposition cross-linking of the air. methylvinylsilicone rubber its The cross-linking the methylvinylsilicone rubber decisively changes the character its cross-linking ofofthe methylvinylsilicone changes of itsofthermal theThe presence of plasticizer in the of low-molecular hydroxydimethylsiloxane, incorporated into thermal decomposition under air.form A small weightrubber loss atdecisively ΔT between 60 and the 395character °C is connected with ◦ C°C thermal decomposition under air. A weight small weight loss ΔT between 60 and connected with with the decomposition air. A loss at ∆Tatbetween 60 and 395395 isisconnected the presence elastomer blend (Scheme 4). form the ofunder plasticizer insmall the of low-molecular hydroxydimethylsiloxane, incorporated into the presence of plasticizer in the form of low-molecular hydroxydimethylsiloxane, incorporated into presence of plasticizer in the form the elastomer blend (Scheme 4). of low-molecular hydroxydimethylsiloxane, incorporated into the the elastomer blend (Scheme elastomer blend (Scheme 4). 4).

n=7 n=7 Scheme 4. Structure of low molecularnplasticizer. =7 Scheme 4. Structure of low molecular plasticizer. Scheme molecular plasticizer. Scheme4.4.Structure Structureofoflow low molecular plasticizer.

Materials 2018, 11, x1298 FOR PEER REVIEW Materials 2018, 11, x FOR PEER REVIEW

9 of 19 9 of 19

The temperature of the beginning of thermal decomposition of cross-linked silicone rubber was Thetemperature temperatureof ofthe thebeginning beginningof ofthermal thermaldecomposition decompositionof ofcross-linked cross-linkedsilicone siliconerubber rubberwas was The increased by 50 °C in relation to the non-cross-linked rubber. Also, the temperature of the maximal increased by 50 °C in relation to the non-cross-linked rubber. Also, the temperature of the maximal ◦ increased by 50rate C in to the non-cross-linked rubber. of the maximal decomposition of relation the cross-linked rubber, amounting to TAlso, RMAX =the 505temperature °C, was increased by 45 °C decomposition rate of the cross-linked rubber, amounting to T RMAX = 505 ◦°C, was increased by 45◦°C decomposition of the cross-linked rubber, amounting to TRMAX = 505 C, was increased by 45 C in relation to therate non-cross-linked rubber. inrelation relationto tothe thenon-cross-linked non-cross-linkedrubber. rubber. in The cross-linking process of SR rubber also caused a considerable reduction in its thermal The cross-linking process of SR rubber rubber also also caused caused aa considerable considerable reduction in in its its thermal thermal The cross-linking process dm/dt of SR decomposition rate. Parameter of non-cross-linked rubber amounts toreduction 16.5%/min, while that of decompositionrate. rate. Parameter dm/dt of rubber to 16.5%/min, while that of decomposition dm/dtThe ofnon-cross-linked non-cross-linked rubberamounts amounts 16.5%/min, while that cross-linked rubber Parameter is 5.25%/min. thermal decomposition residuetowas also considerably cross-linked rubber is 5.25%/min. The thermal decomposition residue was also considerably of cross-linked rubber isto5.25%/min. The thermal decomposition residue was for alsothe considerably increased, increased, amounting 5.25% for the non-cross-linked rubber and 19.95% cross-linked rubber increased, amounting to 5.25% for the non-cross-linked rubber and 19.95% for the cross-linked amounting to 5.25% for the non-cross-linked rubber and 19.95% for the cross-linked rubber Figures 3rubber and 4. Figures 3 and 4. Figures 3 and 4.

Figure 3. TG curves of uncured (SR) and cured (SR OP) silicone rubber. Figure Figure 3. 3. TG TG curves curves of of uncured uncured(SR) (SR)and andcured cured(SR (SROP) OP)silicone siliconerubber. rubber.

Figure 4. DTG curves of uncured (SR) and cured (SR OP) silicone rubber. Figure Figure 4. 4. DTG DTG curves curves of of uncured uncured (SR) (SR)and andcured cured(SR (SROP) OP)silicone siliconerubber. rubber.

The temperature increase in the decomposition of cross-linked silicone rubber in relation to that Thetemperature temperatureincrease increasein inthe thedecomposition decompositionof ofcross-linked cross-linkedsilicone siliconerubber rubberin inrelation relationto tothat that The of non-cross-linked rubber caused a radical mechanism to predominate in the decomposition of the ofnon-cross-linked non-cross-linkedrubber rubbercaused causedaaradical radicalmechanism mechanismto topredominate predominatein inthe thedecomposition decompositionof ofthe the of cross-linked polymethylvinylsiloxane. An increase in temperature caused a hemolytic dissociation of cross-linked polymethylvinylsiloxane. An increase in temperature caused a hemolytic dissociation of cross-linked polymethylvinylsiloxane. An increase in temperature caused a hemolytic dissociation the Si–C bond, which resulted in the formation of macro-radicals whose recombination and crossthethe Si–C bond, which resulted in the of macro-radicals whose recombination and crossof Si–C bond, which resulted in formation the formation of macro-radicals whose recombination and linking decrease the elasticity of the polysiloxane chain and consequently impeded further formation linking decrease the elasticity of the polysiloxane chain and consequently impeded impeded further formation cross-linking decrease the elasticity of the polysiloxane chain and consequently further of cyclic oligomers. of cyclic oligomers. formation of cyclic oligomers. 3.2. POSS Characteristic 3.2. 3.2. POSS POSS Characteristic Characteristic Figure 5 presents TG and DTG curves of the POSS under investigation. Aminopropylisobutyl Figure Figure 55presents presentsTG TGand andDTG DTGcurves curvesofofthe thePOSS POSSunder underinvestigation. investigation.Aminopropylisobutyl Aminopropylisobutyl silsequioxane (AM-POSS) underwent thermal decomposition in a single stage that began at T = 250 ◦ C. silsequioxane thermal decomposition in ainsingle stage thatthat began at T at = 250 silsequioxane(AM-POSS) (AM-POSS)underwent underwent thermal decomposition a single stage began T = 250 °C. The maximal rate of this process at TRMAX = 365 °C amounted to 16.95%/min. A small residue after °C. The maximal rate of this process at TRMAX = 365 °C amounted to 16.95%/min. A small residue after

Materials 2018, 11, 1298

10 of 19

Materials 2018, 11, x FOR PEER REVIEW

10 of 19

The maximal rate of this process at TRMAX = 365 ◦ C amounted to 16.95%/min. A small residue thermal decomposition amounting to 5.75% of the initial sample great after thermal decomposition amounting to 5.75% of the initial sampleweight weightresulted resultedfrom from aa great susceptibility of AM-POSS to sublimation processes. susceptibility of AM-POSS to sublimation processes. Chloroisobutyl POSS POSS (HA-POSS) (HA-POSS) underwent underwent aa two-stage two-stage thermal thermal decomposition. decomposition. The The first first Chloroisobutyl ◦ C, principal stage with a weight lossloss at ΔT 220 and °C,420 whose principal stage of ofthermal thermaldecomposition decomposition with a weight atbetween ∆T between 220420 and ◦ maximal rate at T = 325 °C amounts to 10.03%/min, was connected with the detachment of chloroalkyl whose maximal rate at T = 325 C amounts to 10.03%/min, was connected with the detachment of group, whereas the secondthe stage of thermal recordedrecorded at ΔT between 450 and450 590and °C chloroalkyl group, whereas second stage ofdecomposition thermal decomposition at ∆T between ◦ was connected with the degradation and decomposition of the cage structure. The residue after the 590 C was connected with the degradation and decomposition of the cage structure. The residue after thermal decomposition of HA-POSS amounted to 15.2%, which indicated its its lower susceptibility to the thermal decomposition of HA-POSS amounted to 15.2%, which indicated lower susceptibility sublimation than that of of AM-POSS. to sublimation than that AM-POSS.

Figure 5. 5. Thermal Thermal curves curves of of POSS. POSS. Figure

Vinyl POSS (OL-POSS) underwent a four-stage thermal decomposition (Figure 5). The first three Vinyl POSS (OL-POSS) underwent a four-stage thermal decomposition (Figure 5). The first three decomposition stages, whose maximal rate at T1 = 265 °C, T2 = 320 °C, and T3 = 375 °C amounted to decomposition stages, whose maximal rate at T1 = 265 ◦ C, T2 = 320 ◦ C, and T3 = 375 ◦ C amounted 0.79, 1.80, and 0.36%/min, respectively, were connected with the degradation of unsaturated to 0.79, 1.80, and 0.36%/min, respectively, were connected with the degradation of unsaturated hydrocarbon chains and the detachment of hydrocarbon destruction products. The fourth stage of hydrocarbon chains and the detachment of hydrocarbon destruction products. The fourth stage of thermal decomposition was recorded at ΔT = 450–650 °C as in the case of HA-POSS, which was thermal decomposition was recorded at ∆T = 450–650 ◦ C as in the case of HA-POSS, which was connected with the thermal degradation and decomposition of the cage structure. A very high residue connected with the thermal degradation and decomposition of the cage structure. A very high residue after the thermal decomposition of OL-POSS, amounting to 86.5% at T = 600 °C, resulted from the after the thermal decomposition of OL-POSS, amounting to 86.5% at T = 600 ◦ C, resulted from the susceptibility of this compound to cross-linking and thermal cyclization [27]. susceptibility of this compound to cross-linking and thermal cyclization [27]. FTIR spectra of all studied POSS showed signals at wavenumbers of 745 and 837 cm−1, which FTIR spectra of all studied POSS showed signals at wavenumbers of 745 and 837 cm−1 , which were were connected with stretching vibrations of the –CH group. A peak at wavenumber of 1091 cm−1 connected with stretching vibrations of the –CH group. A peak at wavenumber of 1091 cm−1 was was connected with stretching vibrations of OH group, whereas a signal at wavenumber 1459 cm−1 connected with stretching vibrations of OH group, whereas a signal at wavenumber 1459−1cm−1 corresponded to asymmetric stretching vibration of the –CH3 group. The peak at 2952 cm was corresponded to asymmetric stretching vibration of the –CH3 group. The peak at 2952 cm−1 was attributed to physically connected water at the POSS surface. attributed to physically connected water at the POSS surface. Additionally, the FTIR spectrum of POSS/AM showed signals at 1229 and 1570 cm−1, Additionally, the FTIR spectrum of POSS/AM showed signals at 1229 and 1570 cm−1 , respectively. respectively. The first one was connected with valence vibrations, whereas the second was due to The first one was connected with valence vibrations, whereas the second was due to deformation deformation vibrations of the –NH group. vibrations of the –NH group. IR spectra of POSS/HA showed a sharp signal at wavenumber 560 cm−1 that is connected with IR spectra of POSS/HA showed a sharp signal at wavenumber 560 cm−1 that is connected−1with valence vibration of the C–Cl group. In the IR spectrum of the POSS/OL, a sharp peak at 548 cm was valence vibration of the C–Cl group. In the IR spectrum of the POSS/OL, a sharp peak at 548 cm−1 was observed, which was attributed to vibration of the CH=CH2 group, whereas the signal at 1407 cm−1 observed, which was attributed to vibration of the CH=CH2 group, whereas the signal at 1407 cm−1 corresponded to deformation vibrations of the –CH group in –C=C–H (Figure 6). corresponded to deformation vibrations of the –CH group in –C=C–H (Figure 6).

Materials 2018, 11, 1298 Materials 2018, 11, x FOR PEER REVIEW Materials 2018, 11, x FOR PEER REVIEW

11 of 19 11 of 19 11 of 19

Figure 6. FTIR spectra of studied POSS. Figure 6. FTIR spectra of studied POSS.

3.3. 3.3. IR IR Analysis Analysis of of SR SR Composites Composites 3.3. IR Analysis of SR Composites −1 FTIR signals at at wavenumbers 2963 andand 2891 cm−1cm that FTIR spectrum spectrum of ofsilicone siliconerubber rubbershowed showedsharp sharp signals wavenumbers 2963 2891 −1 that FTIR spectrum ofboth silicone rubber showedand sharp signals atstretching wavenumbers 2963 and 2891 cm were connected with the asymmetric symmetric vibrations of C–H group, that were connected with both the asymmetric and symmetric stretching vibrations of C–H group, −11 were connected were connected withatboth the asymmetric1415, and and symmetric stretching vibrations of C–H group, − respectively. 1258 cm cm stretching respectively. Signals Signals at wavenumbers wavenumbers 695, 695, 1415, and 1258 were connected with with stretching −1 respectively. Signals wavenumbers 695, 1415, and 1258 cm were connected with 3stretching vibrations of the C=Catand groups, and Si–CH groups, vibrations of the C=C and C–H C–H groups, and bending bending stretching stretching derived derived from from Si–CH 3 groups, −1 vibrations of the C=C and C–H groups, and bending stretching derived from Si–CH 3 groups, − 1 respectively strong signals signals at at wavenumbers wavenumbers of of 1077, 1077, 1010, 1010,and and787 787cm cm were connected respectively [28]. [28]. Very Very strong were connected −1 were connected respectively [28]. Very strong signals at wavenumbers of 1077, 1010, and 787 cm with stretching stretching vibrations vibrations of of the the Si–O–Si Si–O–Si group group (Figure (Figure 7). 7). with withThe stretching vibrations ofsilsequioxanes the Si–O–Si group (Figure 7). incorporation of into the silicone not significantly The incorporation of silsequioxanes into the silicone rubberrubber matrix matrix did not did significantly influence The incorporation of silsequioxanes into the silicone rubber matrix did not from significantly 1, influence the of character the FTIR spectrum. into in signal the character the FTIRofspectrum. Taking intoTaking account theaccount increasethe in increase signal from 1410 to 14201410 cm−to −1, in influence thethe character of the FTIR spectrum. Taking into account the increase in signal from 1410 to 1420 cm case of SR/OL vulcanizate, one should state that the silsequioxanes investigated did in the case of SR/OL vulcanizate, one should state that the silsequioxanes investigated did not 1420 cm−1, inwith the case of SR/OL vulcanizate, onethe should state that the silsequioxanes investigated did not combine silicone rubber chains during process of reactive mixing and did not participate combine with silicone rubber chains during the process of reactive mixing and did not participate in not combine with silicone rubber chains during the process of reactive mixing and did not participate in cross-linking of SR rubber. thethe cross-linking of SR rubber. in the cross-linking of SR rubber.

Figure 7. FTIR spectra of SR/POSS composites. Figure Figure7.7.FTIR FTIRspectra spectraofofSR/POSS SR/POSScomposites. composites.

3.4. Thermal Properties of SR Composites 3.4.Thermal ThermalProperties PropertiesofofSR SRComposites Composites 3.4. The incorporation of polyhedral oligomeric silsequioxanes into the silicone rubber matrix The incorporation incorporation of polyhedral oligomeric silsequioxanes into the silicone silicone rubber rubber matrix The polyhedral oligomeric silsequioxanes into matrix decisively influenced theof thermal properties of the rubber, expressed by the temperature of 5% and decisively influenced theTthermal thermal properties ofthe therubber, rubber, expressed bythe thetemperature temperature of 5%and and decisively influenced the properties of expressed by 5% 50% of mass loss (T5 and 50), the thermal decomposition rate of the composite (dm/dt), theof maximal 50% of mass loss (T 5 and T 50 ), the thermal decomposition rate of the composite (dm/dt), the maximal 50% of mass loss (T and T ), the thermal decomposition rate of the composite (dm/dt), the maximal temperature of its thermal decomposition (TRMAX), and the residue after its thermal decomposition 5 50 temperature ofits its thermal decomposition (T RMAX), and the the residue residue after afterits itsthermal thermaldecomposition decomposition temperature of (Pw) and residue atthermal T = 650decomposition °C (P650) (Table (T 3). RMAX (Pw)From andresidue residue T==650 650 (P650 6503, ) (Table 3). that all the silsequioxanes tested brought about an (Pw) and atatT C (P 3). the data listed in◦°C Table it follows From thedata datalisted listed inTable TableT3,53,. itItitfollows followsthat thatall allthe the silsequioxanes tested brought aboutan an From in tested brought about increase in the the value of parameter should, however, besilsequioxanes underlined that the highest increase in increase in the value of parameter parameter however, be increase in increase value of T55.. ItItshould, should, however, beunderlined underlined thatthe thehighest highest increase the valuein ofthe this parameter was recorded for the composites containingthat OL-POSS. In the case of the value of thisand parameter recorded for increased the composites In the case°C of samples SR/OL3 SR/OL6, was parameter T5 was by 200containing °C (sampleOL-POSS. SR/OL3) and by 194 samples SR/OL3 and SR/OL6, parameter T5 was increased by 200 °C (sample SR/OL3) and by 194 °C

Materials 2018, 11, 1298

12 of 19

in the value of this parameter was recorded for the composites containing OL-POSS. In the case of Materials 2018, 11, x FOR PEER REVIEW 12 of 19 samples SR/OL3 and SR/OL6, parameter T5 was increased by 200 ◦ C (sample SR/OL3) and by 194 ◦ C (sample in relation relation to to the (sample SR/OL6) SR/OL6) in the cross-linked cross-linked SR SR rubber rubber (Figure (Figure 8). 8). These These increases increases confirmed confirmed the the susceptibility of vinyl silsequioxane to the processes of thermal cross-linking and cyclization. susceptibility of vinyl silsequioxane to the processes of thermal cross-linking and cyclization.

Figure 8. TG and DTG curves of SR/OL3 composite. Figure 8. TG and DTG curves of SR/OL3 composite. Table 3. Thermal properties of SR/POSS composites. Table 3. Thermal properties of SR/POSS composites. Sample

Sample

T5 (°C)

◦ SR T 5 ( C)

T50 (°C) dm/dt (%/min) TRMAX (°C) T 50 (◦ C) 520 dm/dt (%/min) T RMAX (◦ C) 184 5.25 505

Pw (%)

Pw 45.3(%)

P650 (%)

P650 (%) 44.5

320 520 − 520 49.0 49.544.5 SR SR/AM3 184 5.25 4.74 505 45.3 SR/AM3SR/HA3 320 4.74 5.06 520 49.0 318 − − 525 51.4 51.149.5 SR/HA3SR/OL3 318 − 5.06 525 51.4 384 − 4.87 506 51.3 50.251.1 SR/OL3 384 − 4.87 506 51.3 50.2 SR/AM6 337 579 4.59 524 53.6 51.5 SR/AM6 337 579 4.59 524 53.6 51.5 304 − − 517 52.5 52.352.3 SR/HA6SR/HA6 304 5.40 5.40 517 52.5 378 − − 514 52.3 52.452.4 SR/OL6SR/OL6 378 3.91 3.91 514 52.3 T 5 , T 50 —temperature of sample 5% and 50% mass loss, respectively; dm/dt—maximum rate of thermal T5 , T50 —temperature of sample 5% and 50% mass loss, respectively; dm/dt—maximum rate of thermal decomposition of vulcanizates; TRMAX —temperature of maximum rate of thermal of vulcanizates; decomposition of vulcanizates; TRMAX —temperature of maximum rate ofdecomposition thermal decomposition of Pw—residue after the thermal decomposition of vulcanizates; P650 —residue after heating to T = 650 ◦ C. vulcanizates; Pw—residue after the thermal decomposition of vulcanizates; P650—residue after heating to T = 650 °C.

An important thermal parameter, directly influencing the suppression of the flammability of elastomeric material, is its thermal decomposition rate (parameter dm/dt). of The in the An important thermal parameter, directly influencing the suppression thereduction flammability of destruction rate of polymeric composites exerts a positive influence on decreasing their flammability. elastomeric material, is its thermal decomposition rate (parameter dm/dt). The reduction in the This results rate fromofthe formation of lower exerts amounts of volatile and flammable products of pyrolysis destruction polymeric composites a positive influence on decreasing their flammability. passing to flame, which inhibitsof the freeamounts radical of reactions in the combustion zone. This results from the formation lower volatile proceeding and flammable products of pyrolysis Among the samples tested, the free lowest value of parameter dm/dt shown by SR/OL3 and passing all to flame, which inhibits radical reactions proceeding in was the combustion zone. Among SR/OL6. Thesetested, samples showed values ofwas residue after and all the samples thealso lowest valuethe of highest parameter dm/dt shown bythermal SR/OL3decomposition and SR/OL6. These ◦ C (parameter Pw and P at T = 650also ), which distinctly indicated that vinyl groups of POSS/OL, samples showed the highest values of residue after thermal decomposition and at T = 650 °C 650 ◦ non-active process of reactive mixing and rubber vulcanization carried out at T = 130 C (parameter in Pwthe and P650), which distinctly indicated that vinyl groups of POSS/OL, non-active in the (Figure partmixing in the reactions of vulcanization cross-linking and cyclization during process5), of took reactive and rubber carried out at T proceeding = 130 °C (Figure 5), the tookthermal part in decomposition combustionand of SR rubber composites (Table 3). the thermal decomposition and the reactions ofand cross-linking cyclization proceeding during The values of activation energy of(Table thermal combustion of SR rubber composites 3).destruction of the SR rubber composites tested (Table 4) confirmed that POSS/OL, from amongofall the oligomeric silsequioxanes tested,composites catalyzed the processes The values of activation energy thermal destruction of the SR rubber tested (Table of andPOSS/OL, thermal cyclization to the extent. silsequioxanes tested, catalyzed the 4) cross-linking confirmed that from among allhighest the oligomeric processes of cross-linking and thermal cyclization to the highest extent. Table 4. Activation energy values of SR/POSS composites.

Sample SR SR/AM6 SR/HA6 SR/OL6

Ea (kJ/mol) 46.57 51.84 84.17 98.22

Materials 2018, 11, 1298

13 of 19

Table 4. Activation energy values of SR/POSS composites. Sample

Ea (kJ/mol)

SR SR/AM6 SR/HA6 SR/OL6

46.57 51.84 84.17 98.22

The incorporation of melamine polyphosphate into the silicone rubber matrix, except for parameter T5 , distinctly deteriorated its thermal stability. Under the influence of MPP, not only the thermal decomposition rate, dm/dt, of the composites increased, but also the residue after the thermal decomposition Pw and the residue at 650 ◦ C decreased. Furthermore, melamine polyphosphate considerably reduced the cyclization of silicone rubber as confirmed by lower values of parameter Pw and P650 of SR/MPP composite in relation to the cross-linked SR (Table 5). From the data listed in Table 5, it follows that composites containing synergic MPP system and silsequioxane were characterized by a higher thermal stability expressed by parameters T5 , dm/dt, Pw, and P650 than the SR/MPP composite. A definite reduction in the thermal decomposition rate and an increase in the residue after thermal decomposition and at T = 650 ◦ C of composite resulted from the formation of a ceramic surface layer that effectively blocked the transfer of mass and energy between the sample and flame. Table 5. Thermal properties of SR/POSS/MPP composites. Sample

T 5 (◦ C)

T 50 (◦ C)

dm/dt (%/min)

TRMAX (◦ C)

Pw (%)

P650 (%)

SR SR/MPP SR/AM3/MPP17 SR/AM6/MPP14 SR/HA3/MPP17 SR/HA6/MPP14 SR/OL3/MPP17 SR/OL6/MPP14

184 315 313 305 380 328 340 378

520 420 424 435 418 435 421 440

5.25 17.65 13.32 10.71 16.16 10.08 14.81 9.89

505 408 410 415 412 415 412 415

45.3 32.4 32.8 35.1 33.4 36.2 33.6 35.6

44.5 28.1 29.2 32.9 30.7 33.9 34.2 36.1

The combustion of silicone rubber proceeds in three stages. The first stage is connected with its flameless thermal decomposition, accompanied by the emission of great amounts of gaseous destruction products. The next stage includes their ignition and a slow propagation of flame over the whole sample surface. The final stage of combustion is connected with the glow of combustion residue that assumes a form of a white uneven surface, under which is a brown heterogeneous layer. The incorporation of polyhedral oligomeric silsequioxanes, POSS, into the silicone rubber matrix does not fundamentally change the mode of its combustion; nevertheless, it considerably reduces its flammability. From the test results listed in Table 6, it distinctly follows that the flammability of cross-linked SR rubber, especially in the presence of 6 parts by wt. of AM-POSS, HA-POSS, and OL-POSS was considerably reduced (Table 6).

Materials 2018, 11, 1298

14 of 19

Table 6. Flammability of SR/POSS composites. Sample

TTI (s) ± 5

THR (MJ/m2 ) ± 1

HRR (Kw/m2 ) ± 2

HRRMAX (kW/m2 ) ± 2

EHC (MJ/kg) ± 2

EHCMAX (MJ/kg) ± 1

MLR (g·m−2 ·s−1 ) ± 0.01

SR SR/AM3 SR/HA3 SR/OL3 SR/AM6 SR/HA6 SR/OL6

93 91 80 85 114 106 99

21.6 18.1 20.3 19.3 14.1 20.7 13.5

73.62 59.81 73.59 62.49 52.81 60.58 38.72

124.73 102.67 120.26 105.50 88.72 106.45 66.10

37.22 38.88 36.50 34.41 30.49 38.1 31.72

79.82 77.80 61.50 76.49 61.44 77.42 78.05

0.064 0.049 0.051 0.061 0.057 0.050 0.036

TTI—time to ignition; THR—total heat release; HRR—heat release rate; HRRMAX —maximal heat release rate; EHC—effective combustion heat; EHCMAX —maximal effective combustion heat; MLR—mass loss rate.

In the presence of AM-POSS, the value the maximal heat release rate (HRRMAX ) of SR/AM6 composite was decreased by 28.87% in relation to the cross-linked SR rubber, while it decreased in the case of HA-POSS by 14.65%, and in the case of OL-POSS by 47% (Table 6). Lower values of HRRMAX , but also those of the total heat release (THR), of POSS/SR composite, in relation to the unfilled, cross-linked SR rubber, were connected with the formation of a ceramic coating on the sample surface under combustion (Figures 9 and 10). It should be distinctly stressed that the silica formed during the thermo-oxidative decomposition of SR rubber, despite its high heat capacity, on account of low cohesion forces between its particles, does not lead to the formation of a homogeneous intumescent boundary layer that would efficiently reduce the effectiveness of fuel diffusion to flame and oxygen to the sample surface [1,29]. From the literature review and the test performed, it clearly follows that only in the presence of POSS, the boundary layer formed during the decomposition of silicone composites assumes a form of uniform ceramic coating that effectively reduces the mass and energy flow between sample and flame [11,23]. EDS analyses indicated that in the presence of POSS, the content of silicon increased with a simultaneous decrease in the oxygen content in the combustion residue of POSS/SR composites (Table 7). Table 7. EDS results residue after burning. Sample

C%

Si%

O%

P%

SR SR/AM6 SR/HA6 SR/OL6 SR/MPP SR/AM6/MPP14 SR/HA6/MPP14 SR/OL6/MPP14

−1

51.60 53.87 54.96 55.64 38.27 40.90 42.45 45.36

48.40 46.13 45.04 44.36 49.94 48.40 51.01 49.37

− − − − 1.79 1.11 2.06 2.27

1

−1 −1 −1 10.00 9.59 4.48 3.00

small amount of carbon.

The increase in silicon content confirmed the formation of ceramic structures suppressed in the boundary layer of burning POSS/SR composite, while the decrease in oxygen content in the combustion residue of the composite tested indicated the reduction in oxygen diffusion to the reaction zone and consequently the inhibition of thermo-oxidative processes by the ceramic layer formed during sample combustion. The highest percentage silicon content with the lowest percentage content of oxygen was found in the combustion residue of SR/OL6 composite. The presence of unsaturated bonds in the cage structure of OL-POSS facilitated the cross-linking of residue after thermal decomposition and consequently the formation of a ceramic layer on its surface. The insulating character of the boundary layer of SR/OL6 composite was also confirmed by the parameter of mass loss rate (MLR), that in the case of the composite tested, was lower by more than 43% in relation to the cross-linked SR rubber (Table 6).

Materials 2018, 11, 1298

15 of 19

The effect on the SR/HA composite flammability is also exerted by the chlorine atom present in the HA-POSS structure. Halogens, passing to the gaseous phase of combustion, effectively interrupt the highly energetic radical reactions in both the pre- and flame-zones, consequently decreasing the efficiency of 11, degradation destruction processes of elastomeric composite [30,31]. Materials 2018, x FOR PEER and REVIEW 15 of 19

Figure Figure9.9.HRR HRRcurves curvesof ofSR/POSS SR/POSS composites.

Figure Figure10. 10.THR THRcurves curvesof ofSR/POSS SR/POSS composites.

The incorporation incorporation of of melamine melamine polyphosphate polyphosphate (MPP) (MPP) to to the the silicone silicone rubber rubber matrix matrix caused caused an an The MAX parameters. The increase in the increase in in its its flammability MAX increase flammability expressed expressed by by THR, THR,HRR, HRR,and andHRR HRR MAX parameters. The increase in flammability of cross-linked silicone rubber in the presence of MPP resulted from from two effects. First, the flammability of cross-linked silicone rubber in the presence of MPP resulted two effects. the acidic products of the of thermal decomposition of melamine polyphosphate incorporated into the First, the acidic products the thermal decomposition of melamine polyphosphate incorporated silicone rubber matrix initiated the decomposition of the Si–O bond in the polymer main into the silicone rubber matrix initiated the decomposition of the Si–O bond in the polymer chain. main Second,Second, MPP, increasing the distances between polymeric chains, chains, considerably reducedreduced the thermal chain. MPP, increasing the distances between polymeric considerably the crosslinking process, especially the cyclization of SR rubber as confirmed by both the violent increase thermal crosslinking process, especially the cyclization of SR rubber as confirmed by both the violent in the value ofvalue parameter MLR (Table and the ratethe of thermal decomposition rate, dm/dt, increase in the of parameter MLR8)(Table 8) and rate of thermal decomposition rate, with dm/dt,a simultaneous reduction in parameter T 50 (Table 5). It should be distinctly underlined that the flame50 with a simultaneous reduction in parameter T50 (Table 5). It should be distinctly underlined that the retardant effectiveness of polyphosphates compounds reducing flammability directly depended on flame-retardant effectiveness of polyphosphates compounds reducing flammability directly depended the availability of a carbon source, either in the form of an organic polymer or a carbonizing additive, on the availability of a carbon source, either in the form of an organic polymer or a carbonizing additive, such as as pentaerythritol. pentaerythritol. The The incorporation incorporation of MPP into into the the matrix matrix of of the the poor poor in in carbon carbon SR SR rubber rubber such of MPP caused the the acidic acidic compounds, compounds, formed formed by by the the decomposition decomposition of of melamine melamine polyphosphate, polyphosphate, to to catalyze catalyze caused the decomposition of silicone rubber, practically decreasing the quantity of the homogeneous the decomposition of silicone rubber, practically decreasing the quantity of the homogeneous layer, layer, insulating the carbon residue after the decomposition of SR rubber. insulating the carbon residue after the decomposition of SR rubber. From the data listed in Table 8, it follows that MPP in combination with POSS reduced the flammability of silicone rubber (Table 8, Figures 11 and 12).

Materials 2018, 11, 1298 Materials 2018, 11, x FOR PEER REVIEW

16 of 19 16 of 19

From the data listed in Table 8, it followscomposites that MPPofinSR/POSS/MPP. combination with POSS reduced the Table 8. Flammability flammability of silicone rubber (Table 8, Figures 11 and 12). TTI

THR

HRR

HRR MAX

EHC

EHC MAX

MLR

(s)

(MJ/m2) ± 1

(Kw/m2) ± 2

(kW/m2) ± 2

(MJ/kg) ± 2

(MJ/kg) ± 1

(g·m−2·s−1) ± 0.01

56

24.5

74.04

141.48

27.95

79.13

0.120

SR/AM3/MPP17 TTI56 Sample SR/AM6/MPP14 (s) 63

14.2 THR

42.18 HRR

78.25 HRR

29.49 EHC

60.33 EHC

Sample SR/MPP

SR/MPP SR/HA3/MPP17 SR/AM3/MPP17 SR/HA6/MPP14 SR/AM6/MPP14 SR/HA3/MPP17 SR/OL3/MPP17 SR/HA6/MPP14 SR/OL3/MPP17 SR/OL6/MPP14 SR/OL6/MPP14

56 54 56 63 83 54 70 83 70 72 72

Table 8. Flammability composites of SR/POSS/MPP.

2) ± 1 (MJ/m7.9

24.5 17.4 14.2 12.3 7.9 17.4 11.5 12.3 11.5 9.2 9.2

2) ± 2 (Kw/m 30.47

74.04 69.75 42.18 49.95 30.47 69.75 29.26 49.95 29.26 27.52 27.52

MAX

2) ± 2 (kW/m 55.22

141.48 114.42 78.25 83.58 55.22 114.42 50.12 83.58 50.12 43.63 43.63

(MJ/kg) ±2 13.61

MAX

(MJ/kg) ±1 59.06

27.95 25.31 29.49 22.46 13.61 25.31 21.15 22.46 21.15 14.24 14.24

79.13 77.38 60.33 75.77 59.06 77.38 73.54 75.77 73.54 75.85 75.85

0.052 MLR (g·m−2 ·0.099 s−1 ) ± 0.01 0.120 0.092 0.052 0.085 0.099 0.092 0.049 0.085 0.049 0.059 0.059

SR/AM6/MPP 14, SR/HA6/MPP14, and SR/OL6/MPP14 composites were characterized by lower SR/AM6/MPP SR/HA6/MPP14, and and SR/OL6/MPP14 composites were characterized by values of parameter14, HRR MAX by 60.83, 40.72, 66.97%, respectively, in relation to the SR/MPP lower valuesAlso, of parameter HRRMAXofbymass 60.83, 40.72, 66.97%, respectively, in14, relation to the SR/MPP composite. the parameters loss rateand (MLR) of SR/AM6/MPP SR/HA6/MPP14, and composite. Also,underwent the parameters of mass loss 29.16, rate (MLR) of SR/AM6/MPP SR/HA6/MPP14, SR/OL6/MPP14 reduction by 17.5, and 50.8%, respectively, in14, relation to that of the and SR/OL6/MPP14 reduction by 17.5,of29.16, and 50.8%, POSS/MPP respectively,system in relation to that of SR/MPP composite. underwent The flame retardant action the synergetic expressed by the SR/MPP composite. The flame retardant action of the synergetic POSS/MPP system expressed by parameters HRRMAX, MLR, and THR (Figure 5, Table 8) and EHC (Table 8) resulted from the parameters and THR (Figure Table 8) the anddiffusion EHC (Table resulted fromboundary the formation formation HRR of a ceramic boundary layer that5,reduced of 8) oxygen to the layer MAX , MLR, of a ceramic boundary layer that reduced the diffusion of oxygen to the boundary decreased and decreased the intensity of degradation and thermal destruction processes layer of theand composite, as the intensity of degradation and thermal destruction processes of the composite, as well as constituted well as constituted a barrier for the mass transport, and reduced the quantity of fuel passing to flame a(Table barrier8)for the mass transport, and reduced the quantity of fuel passing to flame (Table 8) [32]. [32]. AAlower lowerflammability flammabilityofofSR/POSS/MPP SR/POSS/MPP composites composites in inrelation relationto tothat thatofofSR/POSS SR/POSS resulted resultedfrom from the synergetic action of MPP and POSS during the formation of an insulating boundary layer during the synergetic action of MPP and POSS during the formation of an insulating boundary layer during the composites. thethermal thermaldecomposition decompositionand andcombustion combustionofofSR/POSS/MPP SR/POSS/MPP composites. The acidic destruction products of melamine polyphosphate initiateinitiate the degradation and thermal The acidic destruction products of melamine polyphosphate the degradation and destruction processes processes of siliconeofrubber, results in the formation very high thermal destruction siliconewhich rubber, which results in the of formation of quantities very high of solid carbon products pass that to the flame an additional sourcesource of fuel quantities of solid carbon that products pass to thezone, flameconstituting zone, constituting an additional of (Parameter HRR , MLR, SR/MPP composite) [33]. The ceramic structure formed in the presence fuel (ParameterMAX HRRMAX, MLR, SR/MPP composite) [33]. The ceramic structure formed in the of POSS, making it making impossible to transfertocarbon destruction products products to flame, to contributes to their presence of POSS, it impossible transfer carbon destruction flame, contributes cross-linking and cyclization, and consequently to increasetothe insulation the boundary to their cross-linking and cyclization, and consequently increase the capacity insulationofcapacity of the layer formed during the decomposition and combustion of SR/POSS/MPP composites. boundary layer formed during the decomposition and combustion of SR/POSS/MPP composites.

Figure Figure11. 11.HRR HRRcurves curvescomposites compositesofofSR/POSS/MPP. SR/POSS/MPP.

Materials 2018, 11, 1298 Materials 2018, 11, x FOR PEER REVIEW

17 of 19 17 of 19

Figure12. 12.THR THRcurves curvescomposites compositesofofSR/POSS/MPP. SR/POSS/MPP. Figure

Conclusions 4.4.Conclusions Thispaper paperpresented presentedflame flameretardant retardantcompounds compoundsfor forsilicone siliconerubber rubberininthe theform formofofpolyhedral polyhedral This oligomeric silsequioxanes (POSS), containing both isobutyl groups and amino-propyl (AM-POSS) or oligomeric silsequioxanes (POSS), containing both isobutyl groups and amino-propyl (AM-POSS) or chloro-propylgroup group(HA-POSS) (HA-POSS)ororvinyl vinylgroups groups(OL-POSS). (OL-POSS).Silsequioxanes Silsequioxaneswere wereincorporated incorporatedinto into chloro-propyl the silicone rubber matrix in a quantity of 3 and 6 parts by wt. by the method of reactive stirring with the silicone rubber matrix in a quantity of 3 and 6 parts by wt. by the method of reactive stirring with theuse useofofaalaboratory laboratorymixing mixingmill. mill. the Basedon onthe theobtained obtained results, results, it it was was found Based found that that the the studied studiedPOSS, POSS,non-active non-activeininthe theprocess processof ◦ reactive mixing and rubber vulcanization carried out at T = 130 °C, took part in the reactions of crossof reactive mixing and rubber vulcanization carried out at T = 130 C, took part in the reactions of linking and cyclization proceeding during the thermal decomposition and combustion of SR rubber cross-linking and cyclization proceeding during the thermal decomposition and combustion of SR composites. rubber composites. VinylPOSS, POSS,among amongallall the polyhedral oligomeric silsequioxanes tested, catalyzed processes Vinyl the polyhedral oligomeric silsequioxanes tested, catalyzed the the processes of of cross-linking and thermal cyclization of silicone rubber. cross-linking and thermal cyclization of silicone rubber. Basedon onthe theSEM-EDX SEM-EDXanalysis, analysis,ititwas wasfound foundthat thatthe thedecrease decreaseofofflammability flammabilityofofsilicone siliconerubber rubber Based in the presence of POSS was connected with the formation, during thermal decomposition in the presence of POSS was connected with the formation, during thermal decomposition of POSS/SRof POSS/SR composites, a ceramic flow massbetween and energy between sample and composites, of a ceramicofcoating thatcoating limited that flowlimited mass and energy sample and flame. flame. The increase in the flammability of cross-linked silicone rubber in the presence of MPP resulted The effects. increaseFirst, in the of cross-linked silicone rubber in the presence of MPP resulted from two theflammability acidic products of the thermal decomposition of melamine polyphosphate from two effects. First, the acidic products of the thermal decomposition of melamine polyphosphate incorporated into the silicone rubber matrix initiated the decomposition of the Si–O bond in the incorporated into the silicone rubber matrixthe initiated the between decomposition of the Si–Oconsiderably bond in the polymer main chain. Second, MPP, increasing distances polymeric chains, polymerthe main chain.crosslinking Second, MPP, increasing the distances between polymeric reduced thermal process, especially the cyclization of SR rubber. chains, considerably reduced the thermal crosslinking process, especially the cyclization of SR rubber. A lower flammability of SR/POSS/MPP composites in relation to that of SR/POSS resulted from A lower flammability of SR/POSS/MPP composites in relation to that ofboundary SR/POSS resulted from the synergetic action of MPP and POSS during the formation of an insulating layer during thethermal synergetic action of MPP POSS during the formation of an insulating boundary layer during the decomposition andand combustion of SR/POSS/MPP composites. the thermal decomposition and combustion of SR/POSS/MPP composites. The acidic destruction products of melamine polyphosphate initiated the degradation and thermal The acidic destruction products melamine polyphosphate initiated thehigh degradation destruction processes of silicone rubber,ofwhich resulted in the formation of very quantitiesand of thermal destruction processes of silicone rubber, which resulted in the formation of very high solid carbon products passing to the flame zone, which constituted an additional source of fuel. quantities solid carbon products passing to the zone, which source The ceramicofstructure formed in the presence offlame POSS, making it constituted impossible an to additional transfer carbon of fuel. Theproducts ceramic structure in theto presence of POSS, making it impossibleand to transfer carbon destruction to flame,formed contributed their cross-linking and cyclization, consequently destruction products to flame, contributed to their cross-linking and cyclization, and consequently to increasing the insulation capacity of the boundary layer formed during the decomposition andto increasing of theSR/POSS/MPP insulation capacity of the boundary layer formed during the decomposition and combustion composites. combustion of SR/POSS/MPP composites. ˙ conceived and designed the experiments. P.R. and B.S. contributed Author Contributions: P.R., B.S., D.B. and W.Z. ˙and Author Contributions: P.R., and B.S.,FTIR D.B. analysis. and W.Ż.D.B. conceived designed SEM-EDX the experiments. B.S. the thermal, cone calorimetry and W.Z. contributed analysis.P.R. P.R.and wrote the paper. contributed the thermal, cone calorimetry and FTIR analysis. D.B. and W.Ż. contributed SEM-EDX analysis. P.R. wrote theThis paper. Funding: research received no external funding.

Funding: This research received no external funding.

Materials 2018, 11, 1298

18 of 19

Conflicts of Interest: The authors declare no conflict of interest.

References 1. 2. 3. 4. 5. 6. 7.

8.

9. 10. 11.

12.

13.

14.

15. 16.

17. 18. 19.

20.

Zhu, C.; Deng, C.; Cao, J.-Y.; Wang, Y.-Z. An efficient flame retardant for silicone rubber: Preparation and application. Polym. Degrad. Stab. 2015, 121, 42–50. [CrossRef] ˙ Rybinski, ´ P.; Zukowski, W.; Bradło, D. Effect of cenospheric fillers on the flammability and fire hazard of silicone rubber composites. J. Therm. Anal. Calorim. 2016, 125, 1373–1386. [CrossRef] Schartel, B. Phosphorous based flame retardancy mechanism old hat or a starting point for future development. Materials 2010, 3, 4710–4745. [CrossRef] [PubMed] Rybinski, ´ P.; Janowska, G. Influence synergetic effect of halloysite nanotubes and halogen-free flame retardants on properties nitrile rubber composites. Thermochim. Acta 2013, 557, 24–30. [CrossRef] Bar, M.; Alagirusamy, R.; Das, A. Flame retardant polymer composites. Fibers Polym. 2015, 16, 705–717. [CrossRef] Balaram, P.; Sirohi, S.; Singh, D. Studies on the effects of various flame retardants on polypropylene. Am. J. Polym. Sci. 2013, 3, 63–69. [CrossRef] ˙ Rybinski, ´ P.; Syrek, B.; Masłowski, M.; Miedzianowska, J.; Strzelec, K.; Zukowski, W.; Bradło, D. Influence of lignocellulose fillers on properties natural rubber composites. J. Polym. Environ. 2018, 26, 2489–2501. [CrossRef] Dong, F.; Zhao, P.; Dou, R.; Feng, S. Amine-functionalized POSS as cross-linkers of polysiloxane containing γ-chloropropyl groups for preparing heat-curable silicone rubber. Mater. Chem. Phys. 2018, 208, 19–27. [CrossRef] Zhang, W.; Camino, G.; Yang, R. Polymer/polyhedral oligomeric silsesquioxane (POSS) nanocomposites: An overview of fire retardance. Prog. Polym. Sci. 2017, 67, 77–125. [CrossRef] Kuo, S.-W.; Chang, F.-C. POSS related polymer nanocomposites. Prog. Polym. Sci. 2011, 36, 1649–1696. [CrossRef] Wang, X.; Hu, Y.; Song, L.; Yang, H.; Yu, B.; Kandola, B.; Deli, D. Comparative study on the synergistic effect of POSS and graphene with melamine phosphate on the flame retardance of poly(butylene succinated). Thermochim. Acta 2012, 543, 156–164. [CrossRef] Liu, Y.; Shi, Y.; Zhang, D.; Li, J.; Huang, G. Preparation and thermal degradation behavior of room temperature vulcanized silicone rubber-g-polyhedral oligomeric silsesquioxanes. Polymer 2013, 54, 6140–6149. [CrossRef] Turgot, G.; Dogan, M.; Tayfun, U.; Ozekoc, G. The effect of POSS particles on the flame retardancy of intumescent polypropylene composites and the structure-property relationship. Polym. Degrad. Stab. 2018, 149, 96–111. [CrossRef] Xuan, S.; Hu, Y.; Song, L.; Wang, X.; Yang, H.; Lu, H. Synergistic effect of polyhedral oligomeric silsesquioxane on the flame retardancy and thermal degradation of intumescent flame retardant polylactide. Combust. Sci. Technol. 2012, 184, 459–468. [CrossRef] Song, L.; Xuan, S.; Wang, X.; Hu, Y. Flame retardancy and thermal degradation behaviors of phosphate in combination with POSS in polylactide composites. Thermochim. Acta 2012, 527, 1–7. [CrossRef] Fox, D.M.; Novy, M.; Brown, K.; Murariu, M.; Harris, R.H.; Seppala, J.E.; Gilman, J.W. Flame retarded poly(lactic acid) using POSS-modified cellulose. Effects of intumescing flame retardant formulations on polymer degradation and composite physical properties. Polym. Degrad. Stab. 2014, 106, 54–65. [CrossRef] Chigwada, G.; Jash, P.; Jiang, D.D.; Wilkie, C.A. Fire retardancy of vinyl ester nanocomposites: Synergy with phosphorous-based fire retardants. Polym. Degrad. Stab. 2005, 89, 85–100. [CrossRef] Gerard, C.; Fontaine, G.; Bourbigot, S. Synergistic and antagonistic effects in flame retardancy of an intumescent epoxy resin. Polym. Adv. Technol. 2011, 22, 1085–1090. [CrossRef] Carosio, F.; Alongi, J. Influence of layer by layer coatings containing octapropylammonium polyhedral oligomeric silsesquioxane and ammonium polyphosphate on the thermal stability and flammability of acrylic fabrics. J. Anal. Appl. Pyrolysis 2016, 119, 114–123. [CrossRef] ˙ Rybinski, ´ P.; Zukowski, W.; Bradło, D. Influence of cenosphere particles on thermal properties composites of silicone rubbers. J. Therm. Anal. Calorim. 2015, 122, 1307–1318. [CrossRef]

Materials 2018, 11, 1298

21. 22. 23. 24.

25. 26. 27. 28.

29. 30. 31. 32.

33.

19 of 19

Flynn, J.H.; Wall, L.A. A quick, direct method for determination of activation energy from thermogravimetric data. J. Polym. Sci. Polym. Lett. 1966, 4, 323–328. [CrossRef] Ozawa, T.A. New method of analyzing thermogravimetric data. Bull. Chem. Soc. Jpn. 1965, 38, 1881–1886. [CrossRef] Hamdai, S.; Longuest, C.; Perrin, D.; Lopez-Cuesta, J.-M.; Ganachaud, F. Flame retardancy of silicone-based materials. Polym. Degrad. Stab. 2009, 94, 465–495. [CrossRef] Radhakrishnan, T.S. New method for evaluation of kinetic parameters and mechanism of degradation from pyrolysis-GC studies. Thermal degradation of polydimethylsiloxanes. J. Appl. Polym. Sci. 1999, 73, 441–450. [CrossRef] Fang, W.; Zeng, X.; Lai, X.; Li, H.; Chen, W.; Zhang, Y. Thermal degradation mechanism of addition-cure liquid silicone rubber with urea-containing silane. Thermochim. Acta 2015, 605, 28–36. [CrossRef] Grassie, N.; Macfarlane, I.G. The thermal degradation of polysiloxanes-I. Poly(dimethylsiloxane). Eur. Polym. J. 1978, 14, 875–884. [CrossRef] Fina, A.; Tabuani, D.; Carniato, F.; Frache, A.; Boccaleri, E.; Camino, G. Polyhedral oligomeric silsesquioxanes (POSS) thermal degradation. Thermochim. Acta 2006, 440, 36–42. [CrossRef] Chen, D.; Liu, Y.; Huang, C. Synergistic effect between POSS and fumed silica on thermal stabilities and mechanical properties of room temperature vulcanized (RTV) silicone rubbers. Polym. Degrad. Stab. 2012, 97, 308–315. [CrossRef] Hshieh, F.-Y. Shielding effects of silica-ash layer on the combustion of silicones and their possible applications on the fire retardancy of organic polymers. Fire Mater. 1998, 22, 69–76. [CrossRef] Zhang, S.; Horrock, A.R. A review of flame retardant polypropylene fibers. Prog. Polym. Sci. 2003, 28, 1517–1538. [CrossRef] Laoutid, F.; Bonnaud, L.; Alexandre, M.; Lopez-Cuesta, J.-M.; Dubois, P. New prospects in flame retardant polymer materials: From fundamentals to nanocomposites. Mater. Sci. Eng. R 2009, 63, 100–125. [CrossRef] ˙ Rybinski, ´ P.; Syrek, B.; Bradło, D.; Zukowski, W.; Anyszka, R.; Imiela, M. Influence of cenospheric fillers on the thermal properties, ceramisation and flammability of nitrile rubber composites. J. Compos. Mater. 2018. [CrossRef] Thirumal, M.; Khastgir, D.; Nando, G.B.; Naik, Y.-P.; Singha, N.K. Halogen-free flame retardant PUF: Effect of melamine compounds on mechanical, thermal and flame retardant properties. Polym. Degrad. Stab. 2010, 95, 1138–1145. [CrossRef] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).