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INVESTIGATION OF PYROLYSIS AND FLAMMABILITY PROCESSES

OF VINYL ACETATE COPOLYMER PAINTS

Mаlik N. Abdikarimov1, Raushan Kh. Turgumbayeva2, Daulet T. Sartayev1

ABSTRACT

Vinyl acetate copolymer paints are applied for fire protection of metal and wooden structures. The processes of com-bustion in a propane/air flame are investigated and inflammable fire-resistant paints based on vinyl acetate-vinyl chloride copolymer including phosphoric acid are developed. It is found that the rate of burning and the weight loss of VA-VC paints are decreased due to the incorporation of alcohol-, oxide-, amine-, phosphorus containing compositions.

Keywords: vinyl acetate, copolymer, paints, pyrolysis, flammability.

Received 17 May 2017Accepted 29 June 2018

Journal of Chemical Technology and Metallurgy, 54, 1, 2019, 48-54

1 Kazakh National Technical Research University after K.I. Satpayev 050022, Almaty, Satpayev str.,22, Almaty, Republik of Kazakhstan E-mail: m.n.abdikarimov@mail.ru2 Kazakh National Pedagogical University after Abai, 050010, Almaty Dostyk str. 13, Almaty, Republik Kazakhstan E-mail: rturgumbayeva@mail.ru

INTRODUCTION

Scientists and practitioners attention is focused on issues of achieving high non-combustible properties of coatings used for fire protection of steel and wooden struc-tures [1 - 5]. Complex physical and chemical processes take place inside and on the surface of the condensed phase in the course of combustion of polymer materials: phase transitions, heat- and thermo-oxidative decomposi-tion, and formation of new phases. Thus, polymers com-bustion is a complex process transforming the original substance in products heated to high temperatures.

Vinyl acetate copolymers are amorphous and non-toxic polar polymers easily soluble in solvents [6]. They are very adhesive to various surfaces (glass, leather, wood, paper), have no color and resistant to light. That is why they are widely used as film-forming materials and adhesives in such areas as construction, varnish and paint manufacturing and printing industry. The develop-ment of fireproof paints is currently an important task due to the flammability of the wooden materials and

synthetic coatings used. It is well recognized that the aggregate stability

(stability) of the aqueous dispersions depends on the particle size, the type of the emulsifiers, the modifiers, stabilizers, the surface-active substances used. There are other factors affecting the surface tension, the adhesion forces and the fire-retardant properties of the paints ap-plied to metal and wooden structures.

The aim of this study is to develop fireproof coatings using copolymers and to investigate their flammability mechanism.

EXPERIMENTALMaterials

Copolymers of vinyl acetate (VA) - vinyl chloride (VC), vinyl acetate - dibutylmaleate (DBM), model DPM 50 35B for printing industry (sector-wised standard OST 6-05-637-78), as well as varnish methyl acrylate (MA) - methyl methacrylate (MMA), chloroprene latex (CR Latex)- trichlorbutadiene (TCB) were used. The additives studied referred to: triethylene glycol (TEG),

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melamine (M), carboxymethyl cellulose (CMC), tricr-esyl phosphate, dinonyl phthalate, sodium tungstate, pentaerythritol (PER), ammonium phosphate monoso-dium, sodium acetate, dioxidiphenylsulfon, guanidine hydrobromide, dicyandiamide, azobisisobutyronitrile, tin(II) chloride, ammonium polyphosphate (APP), am-monium borate, amylopectin, xylitol, urea and nitric acid. The emulsion of poly(vinyl acetate) (PVA) was comparative studied.

MethodsThe experiments carried out were aimed at meas-

uring the weight loss and the kinetic characteristics of samples of vinyl acetate copolymer paints including additives to assess their fire retardant properties as fire proof coatings for metal and wooden substrates, re-spectively. The additives were crushed, sifted through a 0.1 mm mesh and mixed with the paints and varnishes studied in the propeller stirrer. Then 4 mm layer of the each prepared mixture was applied to a metal and a wooden substrate [5] and was left to dry for 48 h.

The samples were subjected to fire testing at the Fire Research Laboratory of the Ministry of Internal Affairs of the Republic of Kazakhstan. The “Ceramic tube” methodology [7] requiring a propane/air flame and a temperature of 1100°C was applied. Chromel-alumel thermocouples of a wire diameter of 200 μm were used to follow the temperature.

The samples thermogravimetric curves were obtained in a specially designed device at a heating rate of 10°C min-1 in the air and an inert gas flow. The thermomechanical curves of 2 ± 0.1 mm platelets were recorded in the designed device at a load of 10 kg cm-2 and a heating rate of 5°C min-1 - 10°C min-1.

RESULTS AND DISCUSSION

Table 1 lists the data on the weight loss and the changes of the burning rate of film-forming vinyl acetate polymer-copolymers applied to various substrates.

The introduction of 36 parts by weight (pbw) of 85 % phosphorus acid results in decrease of the burning

No

Sample Additive,

pbw Ub

gram/min

Vb,

gram/cm2/

min

weight loss,

%

Note:

Substrate

1 Veneer 5.0 0.200 85.0

2 PVA 3.8 0.152 57.6 Metal

3 PVA 4.3 0.172 36.8 Wooden

4 VA-VC 4.3 0.172 42.0 Metal

5 VA-VC 3.1 0.124 36.0 Wood

6 Varnish

MA-MMA 4.1 0.164 50.7 Wooden

7 Latex

CR-TCB 3.0 0.12 41.7 Veneer

8 VA-DBM 5.0 0.200 84.8 Metal

Table 1. Kinetic parameters of samples of paints, emulsions and varnishes under combustion.

where: pbw – parts by weight, Ub- linear burning rate (LBR) and Vb - mass burning rate (MBR).

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rate to 0.8 g min-1 due to the formation of a solid char residue. It is known [1, 2] that phosphorus present in a condensed phase contributes to changing the combustion mechanism of a vinyl acetate-vinyl chloride copolymer. It is determined that chlorine is involved in flame inhibi-tion as well as in tarring of the condensed phase with its further transition to a fine carbonaceous residue. For example, the analysis of VA-VC condensed-phase copolymer+40 pbw of 85 % phosphoric acid shows that 7.25 % content of phosphorus in the original sam-ple is increased to 10.27 % and 17.56 % after burning in a propane/air flame and in an argon flow pyrolysis, correspondingly. The chlorine content of the sample is increased from the original amount of 3.85 % to 16.29 % after burning in a propane/air flame. After pyrolysis in an argon flow its content is 0 %.

The elemental analysis of the carbon residue of VA-VC copolymer paint after combustion and pyrolysis reveals that phosphorus and chlorine remain in the con-densed phase. The corresponding data is shown below:

Polymer P, % Cl, %

1 VA-VC + 40 pbw PA 7.25 3.85

2 After burning in the flames of propane/air burner

10.27 16.29

3 After pyrolysis in an argon flow 17.56 0.00

Phosphorus and its compounds are treated as one of the most effective inhibitors of various polymers combustion and smouldering processes. The effect of phosphorus-containing flame retardants is usually explained as follows. The pyrolysis of polymers con-taining phosphorus compounds results in the formation of phosphoric acid and its anhydrides, which catalyze dehydration and dehydrogenation, and thus contribute to the process of carbonization. Low molecular weight polymers containing polymeric phosphorus-containing flame retardants are used. The latter have better compat-ibility with the base polymer, show less migration and have a higher resistance to various external impacts. There are also studies [6] showing that the incorpora-tion of benzoic, hydrochloric and other acids acceler-ates the release of acetic acid from PVA. Hence, it can be suggested that the introduction of phosphoric acid results in acetate groups’ replacement by phosphoric

acid fragments, which in turn activates the process of carbonization because of the dehydration and dehydro-genation reactions. This can explain the flame inhibition mechanism of the paint based on VA-VC copolymer+40 pbw of 85 % H3PO4. The investigation [8] of the combus-tion of epoxy resins in presence of hydrazine dihydro-chloride shows that the chlorine remaining in VA-VC paint structure also facilitates the process of tarring and its subsequent transformation to a fine-pored char. In general, P+Cl mixture provides a synergistic inhibi-tion of the combustion process of VA-VC copolymer paint+40 pbw of 85% H3PO4. The corresponding scheme reported in ref. [1] is presented in Fig. 1. The first step refers to the acetate group replacement by a phosphoric acid residue. It is accompanied by acetic acid release. This is in agreement with the data of N. Grassie [9 - 11]. The second step is connected with the elimination of hydrogen chloride and the introduction of dihydrogen phosphate anion into the fragmented chain. Thus a com-plete replacement of the acetate and chlorine-containing acid residues is achieved.

The introduction of a mixture of ammonium phos-phate monobasic with dinonyl phthalate or sodium tungstate triggers the poly(vinyl acetate) compositions effervescence on the wooden and metal substrates. This leads to PVA paint burning rate decrease to 3.6 g min-1 and a weight loss of these samples amounting to 53.8 % and 43.0 %, respectively. Sodium tungstate conduces the poly(vinyl acetate) paints effervescence which results in formation of a white fine porous char.

The burning rate and the weight loss of VA-VC paints on wooden substrates show decrease with the introduction of additives such as melamine, pentaeryth-ritol, ammonium phosphate, as well as TEG or sodium acetate.

The burning rate and the weight loss of MA-MMA varnish decrease 4 and 5 times, correspondingly, with the introduction of expanding additives as well as flame retardants.

Chloroprene latex with trichlorobutadiene becomes non-flammable in the presence of phosphoric acid and a foaming agent.

It is found for the first time that sodium tungstate facilitates the emergence of a porous solid fine char while

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the mixture of ammonium phosphate and dioxidiphenyl-sulfon promotes the formation of a fire-resistant coating on the basis of VA-DBM emulsion.

Fig. 2 shows the thermogravimetric curves referring to the application of paints based on VA-VC (36:64 molar %) copolymer with an additive (40 pbw of 85 % phosphoric acid). The presence of phosphoric acid leads to the formation of heat-resistant compounds in the form

of a char residue. Hence, a lag in the decomposition curves is observed.

The comparative thermogravimetric curves referring to the decomposition of VA-DBM, VA-VC and PVA emulsion in an air flow at a heating rate of 10°C min-1 are illustrated in Fig. 3. It is found on the ground of the differential thermal analysis (DTA) that the degradation of VA-DBM copolymer initiates at 80°C.

Fig. 1. Scheme of decomposition of copolymer paints VA-VC by adding phosphoric acid.

Fig. 2. Thermogravimetric curves at a heating rate of 10°C min-1 in the air-flow of paints based on VA-VC copolymer with additives: (1) VA-VC (36:64 mol %); (2) VA-VC + 40 pbw of 85 % phosphor additives.

Fig. 3. Thermogravimetric curves at a heating rate of 10°C min-1 in the air of paint samples: (1) VA-VC; (2) VA-DBM; (3) PVA emulsion.

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As seen from Fig. 3, the run of the curve of PVA emulsion decomposition differs from those of VA co-polymers.

The thermomechanical curves of the paint samples recorded at a heating rate of 5°C min-1 and a load P of 10 kg cm-2 are demonstrated in Fig. 4.

They refer to temperature dependence of the de-formation of VA-VC copolymer including phosphoric acid, pentaerythritol, ammonium phosphate (mono-substituted) and melamine (curve 1). The visco-elastic properties of VA-DBM copolymer (curve 2) are outlined in the temperature interval ranging from 0°C to 90°C. The behaviour illustrated shows that this substance can be used as a polymer matrix for solid surfaces. The in-flection corresponding to 80% strain is observed in the temperature interval of 43°C - 46°C. This corresponds to VA-DBM copolymer glass transition. Three physical states of poly(vinyl acetate), i.e. the vitreous (28°C- 90°C) and those of high-elasticity (190°C - 370°C) and of a viscous-flow (370°C - 430°C) are present according to curve 3.

It is noted that VA-VC copolymer with the addition of phosphoric acid, ammonium phosphate, pentaerythri-tol and melamine, as well as VA copolymer with dibutyl

maleate are subjected to softening at low temperatures (25°C - 27°C). This value is close to that of poly(vinyl acetate) glass transition, i.e. 28°C. When heated, PVA transfers from a glassy to a high-elasticity state show-ing large reversible strains. The processes of hardening intensify with temperature increase. They take place with heat release contributing thus to the deformation processes. The polymer transforms from a high-elasticity to a viscous–flow state. This transformation is accom-panied by a sharp strain increase.

The differential thermal analysis carried out reveals that the thermogram is characterized by exothermic peaks at 30°C (PVA’s glass transition point), 50°C (PVA’s softening point), 230°C, 250°C, 330°C, 390°C, 420°C and 540°C (oxidation), as well as exothermic peaks at 120°C (PVA’s flow temperature), 170°C (deg-radation temperature), 260°C, 300°C, 400°C and 440°C (in-depth pyrolysis). Copolymers of VA-VC and VA-DBM differ from the poly(vinyl acetate) paint by the maximum exo-effect observed at 580°C. Apparently, this is due to the release of a significant amount of heat during the thermal oxidative degradation, which causes the greatest weight loss at this temperature.

It is recognized [12 - 15] that the amorphous struc-ture of poly(vinyl acetate) demonstrates three physical states: vitreous, highly elastic and that of a viscous–flow.

Fig. 4. Thermo-mechanical curves at a heating rate of 5°C min-1 and load of 10 kg cm-2 of water-dispersion paints: (1) VA-VC (100 pbw) + 40 pbw of 85 % phos-phor additives + 15 pbw PER +15 pbw APP + 15 pbw M; (2) VA-DBM; (3) PVA.

Fig. 5. Changing of the temperature of metal substrate during combustion of the samples of VA-VC copoly-mer paint with additives: (1) VA-VC (36:64 mol %); (2) VA-VC (36:64 mol %) + additives(foam agents, spirits, amines, borate compositions); α - the degree of expansion.

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This fact verifies the good correlation found in case of application of thermo-mechanical degradation and dif-ferential thermal analysis.

The fire-retardant properties of PVA and copolymers as well as the effect of the different additives used are reported in refs. [16-23].

Fig. 5 illustrates the curves of metallic substrate temperature rise during the combustion of VA-VC (36:64 mol %) copolymer paint in presence of different addi-tives (foam agents, spirits, amines, borate compositions).

As seen from the Fig. 5, the additives facilitate the formation of spumescent retardant composition based on VA-VC. A fine porous char of an expansion degree of 6 is obtained. Due to its low thermal conductivity the char protects the basic material against the heat flux. It is found that the degree of expansion of paints includ-ing modifying additives amounts to 2-5. The fine char insulates the fire-resistant coating from heat penetration.

CONCLUSIONSThe processes of combustion in a propane/air flame

are investigated and inflammable fire-resistant paints based on vinyl acetate-vinyl chloride copolymer includ-ing phosphoric acid are developed.

It is found that the rate of burning and the weight loss of VA-VC paints on a wooden substrate are decreased due to the incorporation of alcohol-, oxide-, amine-, phosphorus containing compositions.

It is determined that the burning rate and the weight loss of MA-MMA varnish decrease 4 and 5 times, cor-respondingly, with the introduction of foaming additives and combustion inhibitors.

Chloroprene latex with trichlorbutadiene becomes inflammable in the presence of phosphorus containing compositions and foaming agents.

Sodium tungstate causes generation of a stable fine char.

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