RELAXATION MAXIMUM OF INTERPENETRATING POLYMER NETWORKS FROM EPOXY AND POLYURETHANE POLYMERS

ABSTRACT

The results of a study of the temperature dependence characterizing the viscoelastic behavior of interpenetrating polymer networks (IPN) compositions based on epoxy and polyurethane polymers are presented, where it allows studying and analyzing the dynamic -mechanical spectra of IPN relaxation maxima.

АННОТАЦИЯ

Приводятся результаты исследований температурной зависимости, характеризующей вязкоупругое поведение взаимопроникающих полимерных сеток (ВПС) композиций на основе эпоксидных и полиуретановых полимеров, что позволяет изучить динамическо-механические спектры ВПС релаксационных максимумов.

Keywords: Relaxation maximum, polyurethane, viscoelastic properties, interpenetrating polymer networks.

Ключевые слова: Релаксационный максимум, полиуретан, вязкоупругие свойства, взаимопроникающие полимерные сетки.

Introduction. The viscoelastic behavior of polymers determines a unique set of their basic physical and mechanical properties. This behavior is most pronounced in the region of transition of polymers from the glassy to the highly elastic state, where diffusion displacements of chain segments, referred to as relaxation, are observed. On the temperature dependence of the mechanical loss coefficient, five regions of viscoelastic behavior of polymers are observed, due to a certain type of molecular mobility: vibrations of atomic groups (γ-relaxation) correspond to the glassy state, diffusion movement of price segments (α-relaxation) corresponds to the transitional state; highly elastic - fast short movements, slow long-range movements (γ-relaxation); viscoelastic flow - slippage of the long-range engagement (δ-transition); viscous flow - the movement of macromolecules as a whole (δ-relaxation) [1-8].

The length of the plateau of the transition region, where relaxation occurs, strongly depends on the length of molecular chains, which is sometimes used to determine the molecular weight of polymers [1].

The molecular mobility of polymers, in addition to the influence of temperature, is most pronounced in cases where the exposure time is comparable to the system relaxation time [4].

The processes of mechanical relaxation in polymers are subdivided into linear and non-linear. Linear relaxations of polymers are not associated with phase structural changes in polymers, but are determined only by the rearrangement of molecular regions when an equilibrium stress is established. Nonlinear relaxations of polymers are observed at high stresses and strains and are associated with structural changes in polymers. The processes of nonlinear relaxation in amorphous and crystalline polymers have much in common, which makes it possible to associate them with the presence of supramolecular formations in solid polymers [ 6, 7].

Change morphology of supramolecular structures, as well as their transformation as a result of destruction in the process deformation of the samples, causes the manifestation nonlinear relaxation properties [ 5-7].

The absence of coherent molecular theories describing relaxation phenomena in polymers makes the experimental study of the processes of molecular relaxation of polymers in a wide temperature range especially important, which is the aim of the study in this article.

The aim of the study is to study the maximum relaxation of IPN from epoxy and polyurethane polymers.

Object and method of research. The object of the study is EPS based on epoxy polymers and polyurethane. The choice of IPN is due to the fact that the presence of flexible thermoplastic polymer macromolecules makes it possible to sharply increase the viscoelastic properties of the system. When choosing epoxy polymers as a component of the system, their manufacturability, high adhesion to the substrate, high dynamic modulus of elasticity and strength characteristics were taken into account. Instead, epoxy resins have relatively low mechanical loss and impact strength.

Based on the foregoing, EPS based on epoxy polymers (from epoxy oligomers ED-16, ED-20 (state standard 10587-76), EIS-1 (technical conditions 38 1091 76) cured with maleic anhydride (state standard 57-58 -78) and thermoplastic polyurethanes UK-1 and thermoplastic PR grade (technical conditions 38-103-185 -92). The choice of maleic anhydride hardener is due to high-temperature curing of epoxy oligomers in the presence of thermoplastic polyurethane in the system. Polyurethane dissolves in epoxy oligomers at elevated temperatures.

As fillers, kaolin (state standard 6138-76) from the Angren deposit, talc (state standard 879-76) from the Zavalyevskoye deposit, and graphite (state standard 44404-78) were chosen, which have a lamellar structure, which contribute to an increase in mechanical losses due to internal friction between particles and lead to a significant reduction in the cost of the composition.

This section discusses methods for determining the most important physical and mechanical properties of composite polymer materials.

The adhesion strength of coatings based on the studied compositions was determined by the method of detachment of fungi connected to each other with a binder on an FP-100/1 (GDR) tensile testing machine.

The microhardness of coatings based on the studied polymers and compositions based on them was determined using a PMT-Z instrument, which is a microscope with a device for pressing an indenter into the test material under a certain static load.

Microhardness H_M was calculated by the formula:

                                                             (1)

where: K - instrument constant, equal to 1854;

P - static load N_M;

d is the diagonal of the imprint formed by indentation in the coating under a certain static load, M.

When determining the values of microhardness, it is important in a strictly controlled range to maintain the load value P, the load time τ and the duration of the load τ _for. When determining microhardness, we took:

P=1 H;        τ=15s; τ_{duration =} 30 s

The impact strength of polymer coatings was determined on a U-2 device, which is a vertical impact tester.

The device consists of a frame, a striker anvil pressed into it, a guide pipe, weights and devices for dropping the load.

The coated plate to be tested is placed between the anvil and the striker ball. The load in the guide tube can be installed at any height up to 0.5 m. The impact strength of the coating (in N * m) was evaluated by the maximum height from which a load weighing 19.6 N falls without causing mechanical destruction of the coating: delamination, cracking, chipping and deformation at the impact site.

The coating thickness was measured with a TIP-10 magnetic thickness gauge.

To study the thermophysical properties, in particular, the determination of the thermal conductivity coefficient, we previously applied the stationary method for determining the heat flux at room temperature - the thermometric method.

The method is based on recording the heat flux from a flat source of constant power through the test sample to a body with a constant temperature.

The temperature dependence of the thermal conductivity of the studied polymers and compositions based on them was studied by the dynamic method.

Experimental determination of the thermal conductivity of the studied compositions was carried out in the temperature range (240-500 K) according to State standard 23630.2-79 on the IT - λ - 400 device, one of the few devices for thermophysical research produced by our industry.

The thermal conductivity was measured on samples of polymer compositions λ 15 mm.

The thermal conductivity of the samples for each temperature was calculated by the formula:

                                                                                 (2)

Where. h- height of the sample: P₀ thermal resistance of the sample. The thermal resistance of the sample was calculated by the formula [9-10 ] .

                                                             (3)

where:  ₀ - sample temperature every 25K;

P_T - temperature on the heat meter, expressed in divisions of the measuring device;

S - cross-sectional area, m²;

P_K is the correction that takes into account the thermal resistance of the contacts and is determined from the calibration of the instruments;

K_T - coefficient of proportionality characterizing the effective conductivity of the working layer, which is determined from the calibration of the device;

K_C - correction for the heat capacity of the sample, determined by the formula:

                                                            (4)

Here: C _o is the specific heat capacity of the sample, J/(kg.K);

Сс - specific heat capacity of the rod, J / (kg.K)

M_s - mass of the sample, kg;

M_r  - is the mass of the rod, kg.

Processing of all experimental data was carried out according to known methods of mathematical statistics.

Research results and their analysis. Currently, thermoplastic polyurethanes, products of the interaction of polyethers and polyesters with various diisocyanates, are increasingly used in various industries. Their characteristics are close to high-modulus rubber-like materials and have high circuit flexibility and a particularly high mechanical loss factor.

The use of thermosetting epoxy polymers paired with a thermoplastic polymer, in particular polyurethane, gives compositions based on them high physical and mechanical, impact-strength and especially viscoelastic properties, which makes it possible to obtain effective vibration-absorbing, soundproofing and damping materials for use in mechanical engineering and other industries.

As is known, the physical and mechanical, including the viscoelastic properties of any polymer, the compositions based on them significantly depend on the ambient temperature.

The study of the temperature dependence of the index, which characterizes the viscoelastic behavior of EPS compositions based on epoxy and polyurethane polymers, and the mechanical loss coefficient makes it possible to determine an important operational characteristic, its glass transition temperature, where the main relaxation process occurs due to the segmental mobility of the chains.

The interpenetration of structures carried out at the supramolecular level and the resulting changes in the structure of the polymers that make up the system during the formation of a new structure should undoubtedly affect the segmental mobility of the chains, and hence their glass transition temperature.

Figure 1 shows the results of studying the dependence of the relaxation maxima of the ED:PU system on their glass transition temperature.

As can be seen from Fig. 1, the dynamic mechanical spectra of the IPN of relaxation maxima showed that the glass transition temperatures of the homopolymers that make up the system shift and approach each other.

The maxima of the mechanical loss coefficient of the components somewhat decreases to the ratio ED:PU 70:30.

The observed shift in the position of the glass transition temperature of the components of the system and their approach to each other is a consequence of the mutual penetration of the structures. Macromolecules of an epoxy polymer, or rather its network structure, serves as a filler for polyurethane.

Consequently, the glass transition temperature of the polyurethane component of the EPS will shift towards higher temperatures. Flexible thermoplastic polyurethane macromolecules play the role of a plasticizer for the epoxy polymer, and also contribute to an increase in defects in the spatial network of the rigid component. These circumstances lead to a shift in the glass transition temperature of the epoxy polymer towards lower temperatures. The greater the content of polyurethane in the system, the greater the imperfection of the epoxy network and its glass transition temperature shifts towards lower temperatures. At a mixture ratio of 65:35 (ED:PU), the glass transition temperature of polyurethane shifted towards lower temperatures so much that it turned out to be lower than the glass transition temperature of pure polyurethane (Fig. 1). This is due to the destruction of large supramolecular structures of polyurethane and a decrease in the packing density of macromolecules in the system.

A sharp increase in the mechanical loss coefficient of EPS as a whole, starting from the component ratio of 65:35 (ED:PU), is due to an increase in the interaction between macromolecules and supramolecular structures of the homopolymers that make up the system, in the absence of a chemical bond between them.

At a component content of 40:60 (ED:PU), the interaction between macromolecules of homopolymers decreases due to the discontinuity of the epoxy phase, therefore, the value of the mechanical loss coefficient decreases, and some increase in the value of the dynamic modulus of elasticity of the system is observed.

Figure 1. Component ratio: 1-PU-100%, ED-100%; ED:PU 2- 85:15; 3-75:25; 4 - 65:35; 5 - 60:40; 6 - 50:50; 7 - 30:70

L- total length of segmental mobility of chains

l_{1 -} length sluggish epoxy resin

l_{2 -} length mobility of polyurethane

h₁  - the height of the relaxation peak of the epoxy polymer

h2 - the height of the relaxation peak of polyurethane

Thus, it can be argued that in this metal-polymer system there is an interweaving of macromolecules of two heterogeneous polymers and properties that differ significantly from the properties of the polymers that make up the system.

Experiments have shown that the same conclusion can be reached by analyzing not only relaxation but also resonance maxima.

Conclusion. An analysis of the results of studies in the field of dynamic mechanical spectra of IPN of relaxation maxima showed that the glass transition temperatures of the homopolymers that make up IPN shift and are attached to each other, which is a consequence of the mutual penetration of structures.

Thus, the simultaneous action of these mechanisms leads to an extreme dependence of the properties of the mixture on the concentration of the introduced component, which, in turn, leads to an acceleration of relaxation processes in the epoxy polymer-polyurethane system and coatings based on them.

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