The Development of a Functional Additive Based on a Nanosized Filler of Natural Origin for the Production of thin Multilayer Heat-Shrink Films

Abstract

This paper sets out the results of developing a functional additive based on low-density polyethylene and a copolymer of ethylene with butene, hexene, and propylene to modify the properties of flexible polymer packing by mean of the controlled effect of a concentrate of nanosized filler introduced during polymer processing with the aim of replacing imported filler concentrates.

Keywords

low-density polyethylene copolymer of ethylene with butene hexene propylene nanosized filler diatomite functional additive heat-shrink film

An urgent task in the field of the production of thin heat-shrink films is to develop a procedure for controlling their properties by introducing functional additives. The effect of nanosized fillers on the properties of polymers used in film production has been described elsewhere [1,2], but this has not been studied sufficiently extensively for multilayer films.

The investigation incorporates the development of an effective filler concentrate intended for introduction into polymer feedstock in the production of multilayer heat-shrink films.

The method for modifying the surface layers of heat-shrink film with a functional additive was based on the considerable influence of the supermolecular structure, and also the conditions under which relaxation processes take place in the polymer melt, which influence the properties of the obtained multilayer heat-shrink films when used in high-speed packaging lines [3,4].

The effect of small quantities of modifiers and concentrates of fillers on the properties of polyethylene has been described in various studies [5–8], in which the method for selecting the modifiers and the main concentration dependences in the creation of filler concentrates are set out in detail.

Concentrates of functional additives contain a large amount of low-molecular-weight filler in the polymer binder (low-density polyethylene and a copolymer of ethylene with butene, hexene, and propylene). When nanosized filler is introduced into the polymer matrix, the polymer melt viscosity increases, and therefore, to produce a filler concentrate, it is necessary to use plasticisers that increase the melt flow index of the concentrate.

In the present work, plasticisers and modifiers of the deformation and strength properties of the end product – heat-shrink film – were used [1].

Modification during processing has a complex effect on the structure and properties of the polymers, and here the change in properties of the polymers is not proportional to the content of modifying component [9]. The observed effect of modifiers on the structure of the polymer formed, governed by non-additive change in its rheological and relaxation properties, seems to be the reason for improvement in the entire combination of properties of the articles formed [10]. Markov and Kuleznev [11] established that the modifiers, not possessing compatibility with the polymers, are localised in the interstructural region of the polymers and promote growth in the mobility of the continuous chains of macromolecules in the amorphous disordered zones and increase in the total mobility of all supermolecular elements in the process of structure formation.

Diatomite was used as the nanosized filler. Diatomite (kieselguhr, trepel) is a sedimentary rock consisting of the remains of diatomic algae and the simplest organisms. Diatomite is composed of 86% silica (SiO₂). Only 8% of the diatomite obtained worldwide is used as a functional filler for composite materials.

In the present investigation, diatomite of the Inzensky field was used. Its recovery, comminution, drying, and roasting were carried out at PK Kvant.

The low content of metals in ionised form on the surface of the filler particles rules out their interaction with the polymer melt and the early ageing of articles manufactured from the polymeric material.

The uniqueness of the filler consists in its propensity to disperse (to break down during high-speed mixing), which determines the high quality of distribution and the small increase in melt viscosity of the composite and the possibility of introducing pigments and other functional additives.

The physicomechanical characteristics of the obtained specimens were assessed by the standard procedures.

The Preparation of the Composite and Specimens for Tests

Composites based on low-density polyethylene (LDPE) and a copolymer of ethylene with butene, hexene, and propylene (CEBHP) were produced by melt mixing in a twin-screw extruder of the Chinese company Jiangsu Xinda Science & Technology (screw diameter 20 mm) at temperatures of 120–200°C/120–225°C and screw speeds of 130 rev/min.

The test specimens were manufactured by injection moulding on a Politest machine (Ray-Ran) with an injection cylinder temperature of 210–220°C, a mould temperature of 60°C, and a pressure of 8 bar.

The thermophysical properties of the mixes based on LDPE and CEBHP were investigated by differential scanning calorimetry (DSC) on a PerkinElmer DSC 4000 differential scanning calorimeter in air with a heating rate of 10°C/min.

The grain size distribution and mineral composition of the filler were investigated.

The grain size distribution of filler specimens is given in Figure 1.

Tests showed a narrow size distribution of filler particles, which enables this type of filler to be classified as nanosized.

The chemical composition of the introduced filler is shown in Table 1 .

Table 1

The results of mineral analysis of specimens of diatomites produced by PK Kvant

Specimen	Quartz	Plagioclase (albite)	Diatomite	Smectite (calcium montmorillonite)	Illite
8_2709-FM	2.5	0.0	88.1	5.9	3.5
9_2709-FB	2.7	0.0	82.5	12.1	2.7
15_2809-FM	2.6	0.5	87.4	3.3	6.2
16_2809-FB	2.0	1.3	88.4	7.0	1.3

The low value of the free surface energy of the filler, which can be controlled at the stage of diatomite production, ensures excellent compatibility with the polymer matrix melt.

The physicomechanical and rheological properties of composites based on LDPE and CEBHP are given in Table 2 .

Table 2

The physicomechanical and rheological properties of composites based on LDPE and on CEBHP

	LDPE 15303-003	LDPE 15303-003 + diatomite, %			CEBHP, %	CEBHP + diatomite, %
	LDPE 15303-003	90 + 10	80 + 20	70 + 30	CEBHP, %	90 + 10	80 + 20	70 + 30
Izod notched impact strength, kJ/m²	37.53	42.43	45.9	34.73	42.3	46.45	47.2	33.3
Elastic modulus in bend, MPa	167.25	180.3	233.6	305.6	227.6	293.5	337	356
Tensile elastic modulus, MPa	182	189	276	413.6	287.4	391.5	497	599
Yield point, MPa	8.2	9.3	7.72	7.3	9.2	9.3	7.9	7.2
MFI,^a g/10 min (at 2.16 kgf and 190°C)	0.3	0.179	0.15	0.12	0.55	0.38	0.29	0.27
Elongation at break, %	164	166	123	84	1130	1138	908	642

MFI – melt flow index.

It can be concluded from Table 2 that the elastic modulus of specimens of composite materials is raised considerably when nanosized filler is introduced, and this is accompanied with an insignificant reduction in the mechanical characteristics.

The introduction of solid filler particles is always accompanied with an increase in the elastic modulus of the materials. As the filler particles have a considerably higher rigidity than the matrix, i.e. are hardly deformed when a load is applied, they also limit the deformation of the polymer, which leads to an increase in the elastic modulus of the entire composite ( Figures 2 and 3 ).

In Figures 2 and 3 , an increase in the elastic modulus when 10% filler is introduced can be seen, and here the highest elastic modulus values are possessed by composites filled with 30% diatomite and with a matrix of CEBHP. In all likelihood, the shape of the particles in the form of nanosized diatomite plates has a reinforcing effect in the case of the filling of CEBHP – they impart a considerably higher elastic modulus.

The yield point (presented in Figure 4 ) decreases when 20 and 30% diatomite are introduced, both for LDPE and for the copolymer of ethylene with butene, hexene, and propylene.

The diagram presented in Figure 4 indicates a reduction in the strength of the material, which evidently is due to the aggregation of filler particles, which are stress concentrators. When 10% diatomite is introduced, the yield point of the composite materials is not lowered, and higher adhesion between the matrix and filler seems to occur.

The most significant indices of increase in the impact strength of the composites ( Figure 5 ) are the ultradispersity and the concentration of particles.

Composites with 20% diatomite filler (for LDPE and CEBHP) are characterised by higher values of notched impact strength by comparison with filler concentrations of 10 and 30%. Evidently this is due to the orientation of the diatomite particles. When the direction of the impact load coincides with the direction of orientation of the filler particles, they more effectively prevent the nucleation and propagation of cracks in specimens and promote a more uniform distribution of impact load in the composite material. The impact strength and elastic modulus are significant indices of the service properties of the flexible polymer packing, as they determine the puncture resistance and non-stretchability of the film during service.

At the next stage of the study, we investigated the effect of the filler on the thermophysical and structural indices of the composite materials.

The results of processing DSC data for mixes based on LDPE and CEBHP are given in Table 3 .

Table 3

The results of processing DSC data for blends based on LDPE and on CEBHP

Composition (LDPE/CEBHP + diatomite)	T_max.melt,°C		ΔH_melt, J/g		Degree of crystallinity of LDPE, %	Degree of crystallinity of CEBHP, %
Composition (LDPE/CEBHP + diatomite)	LDPE	CEBHP	LDPE	CEBHP	Degree of crystallinity of LDPE, %	Degree of crystallinity of CEBHP, %
100:0	109.84	126.8	61.83	85.81	36.7	42.4
90:10	108.54	126.7	64.66	72.49	36.3	42.3
80:20	108.06	125.7	50.01	63.32	36.1	42.0
70:30	108.70	125.9	48.36	60.69	36.3	42.1

Table 3 gives the results of DSC analysis of experimental specimens, which showed that changes in the melting temperature were minimal for all specimens, within the limits of measurement error, from which it follows that the introduction of the nanosized filler does not lead to any marked change in the temperatures of phase and physical states of composites based on LDPE and CEBHP or change in the degree of crystallinity.

An analysis of the values of the heat flux during melting of composites, ΔH, showed its systematic reduction with increase in the content of fillers in the polymer matrix.

The obtained data can be explained by the fact that particles of solid filler lie mainly in the interstructural regions of the polymeric material, and are not heterogeneous crystallisation centres. Therefore, when nanosized powder is introduced into the polymer matrix, there is no marked increase in the degree of crystallinity of the polymer.

Experimental specimens of polymer composites are characterised by a degree of crystallinity very similar to that of the matrix polymers (for LDPE/CEBHP) and also a very similar melting temperature, which indicates the absence or insignificance of polymer degradation processes in the method used for composite production. The obtained melting temperature values of the polymer composites indicate that for all the experimental specimens they lie in the admissible range.

The conducted investigations showed the following:

•

The developed formulations of concentrates of nanosized filler of natural origin are effective for modification of the properties of low-density polyethylene and a copolymer of ethylene with butene, hexene, and propylene intended for the production of heat-shrink films to be used in automated packaging lines.

•

The introduction into LDPE of a nanosized filler of natural origin – Inzensky field diatomite – will lead to an improvement in the service properties of heat-shrink films.

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