Plasma treatment before filling particulate plastic polymers into the mold

Abstract

Particle foam process for processing particulate plastic polymers (1) in a forming tool (5, 6, 7, 8, 9) with one or more cavities (5) and one or more injector nozzles (2) for injecting the particulate plastic polymers (1) into the cavity or cavities (5), characterized in that the particulate plastic polymers (1) are fed into a plasma unit (3) through a metering valve (18) and then an inlet pipe (19) before being injected into the cavity, in which they are surrounded by plasma by at least two opposing plasma nozzles (10) arranged at an angle of 1 to 90° relative to the inlet pipe (19), from where they are pumped into a particle container (4) and collected there,from there into an injector nozzle (2) with a second metering valve (20) located in front of the injector nozzle (2) by means of an air venturi pump (12) located there by means of negative pressure and from there also with the aid of the air venturi pump (12) into the tool (5, 6, 7, 8, 9).

Description

[0001] The invention relates to a method and a device for plasma pretreatment of particulate plastic polymers (PKP) before entering the cavity of a forming tool.

[0002] In plastics processing, molded parts are manufactured in molds using plastic polymers, which are usually supplied by the chemical industry as granules, powder, films, or sheets. The production of molded parts from amorphous materials is called primary molding. This process is particularly important for thermoplastics. Post-processing is usually unnecessary. For thermosets, only primary molding processes are applicable.

[0003] During injection molding, the liquefied plastic is injected into a cavity, where it is first compressed and then cooled. The molded part is then ejected. Back-injection molding is a special processing technique. This process produces molded parts consisting of a thermoplastic carrier and another material. The material, which can be a film or textile, is back-injected with plastic.

[0004] Foam particles made of polystyrene, polypropylene, or polyethylene, for example, can be used in the molding process to produce foam molded parts (so-called particle foaming). Foam beads with diameters of approximately one to eight millimeters are blown into cavities via injector nozzles and bonded together using, for example, steam. After a cooling and stabilization phase, the molded parts can be removed.

[0005] The treatment of plastic surfaces is necessary for many applications because plastic surfaces are normally inert and, in particular, not wettable with liquids, and therefore do not accept printing ink or adhesives, for example. Treatment changes the surface structure of the plastic so that the surface becomes wettable for liquids with a relatively high surface tension. Various surface activation methods for plastics are known. One method is corona discharge. Here, the plastic to be treated is passed between two electrodes coated with ceramic material, to which a high-frequency high voltage is applied, so that a corona-shaped discharge occurs through the plastic material. This method is suitable for relatively thin workpieces with a smooth surface, such as films.

[0006] In pure plasma surface treatment, by applying an electrical voltage to an electrode in a chamber, some of the atoms in air or argon are ionized, thus igniting a plasma. The ions in the plasma strive to become electrically neutral again, i.e., to acquire another electron. The reactivity of the ions is so strong that electrons are removed from chemical bonds in the polymer of the plastic. This results in open bonds and thus wettability. A similar result can be achieved using ozone as an activation gas. When UV radiation with a wavelength below 200 nm strikes oxygen in the air, ozone is formed. Numerous series of measurements have shown that the activation of plastic surfaces is promoted by the use of this formed ozone. A disadvantage of pure ozonation is the formation of unknown and potentially toxic products.

[0007] Activation allows for easy bonding with materials other than those conventionally used in molds. Above all, the use of antibacterial materials is also possible. The use of antibacterial surfaces is becoming increasingly important in many industrial sectors. However, their production involves considerable effort.

[0008] Plasma-enhanced chemical vapor deposition (PE-CVD) can also be used to apply chemicals to the plastic surface. These can be layers made of, among others, diamond, silicon nitride, silicon dioxide, metal / silicon hybrids (silicides), titanium nitride, tin oxide, silicon carbide, or copper, zinc, and silver. A solid component is deposited on the heated surface of a substrate as a result of a chemical reaction from the gas phase. This requires the existence of volatile compounds in the layer components that deposit the solid layer at a specific reaction temperature. The creation of antibacterial surfaces using silver, copper, and zinc is known from Gerullis et al., among others. These materials are subsequently applied to the surfaces in a complex process using silicon oxide films.

[0009] Plasma post-treatment directly in the mold is known from the prior art. It involves introducing a plasma stream into the cavity of the semi-finished product or final product produced by forming or primary forming. US5236636A describes such a plasma post-treatment in the molded part. The method described involves injection molding an object in the molded part, moving the mold apart after injection molding, creating a vacuum, and introducing an ionizable gas into the space. In EP3277475B1, a cavity is opened in the injection mold, into which a plasma jet generated by the plasma nozzle is introduced. In this way, the movement of the core can specifically expose an area for plasma treatment. The effort required to open the mold, create a vacuum, and generate a plasma outside or inside the mold is enormous. Molds are generally designed for specific shapes.This would require the technology to be redesigned and implemented for each mold. Furthermore, typically only individual areas of the finished mold can be treated, not the entire part, because the molds' shape does not allow for comprehensive treatment. In DE202016000466U1, a rigid plastic insert is activated by plasma and then back-injected with PU foam. The disadvantage of this technology is that multiple process steps are required to produce the finished part.

[0010] JP2010113838A describes a method for treating insulating particles and insulating powder to prevent static charging. For this purpose, a passage to an injection-molded part is disclosed, without specifying the presence or number of cavities, in which a corona discharge with neutralization takes place on the glass housing, starting from a pointed electrode.

[0011] DE102016106972B4 shows a plasma generator on a filling injector that opens directly into the mold. Before being injected into the cavity, no particulate plastic polymers enter a plasma unit through a metering valve and then an inlet pipe. A separate metering valve is not present. They are also not subjected to plasma flow through at least two opposing plasma nozzles arranged at an angle of 1 to 89° relative to the inlet pipe. Rather, the purpose of the described setup is to heat the foam particles.

[0012] DE102012004385A1 describes the welding of dissimilar thermoplastics by prior functionalization of the joining surfaces through layer-forming or non-layer-forming plasma processes. However, the process does not describe how an effective implementation can be achieved, particularly in standard forming or primary forming processes. EP986939B1 discloses an eccentrically arranged plasma nozzle for generating a plasma jet directed parallel to the rotation axis. The disadvantage is that no treatment of particulate plastic polymers prior to use in a mold is described, thus limiting the area of ​​application to the plastic surfaces of certain already formed parts. Furthermore, the large-scale post-treatment poses a risk to occupational safety, as ozone and high electric currents can damage the health of workers.

[0013] Ozone treatment of plastics in the cavity is known from DE102004061268A1. However, this separate second treatment step is disadvantageous from a process economics perspective and also affects already molded parts.

[0014] DE102016120781A1 describes the plasma deposition of an organosilicon layer for improved adhesion of plastic surfaces. The production of antibacterial plastic surfaces is known from DE1694483A. This document discloses that an already molded plastic is mixed with an antibacterial substance containing an alkyl chain in the molecule, and the bonding occurs at elevated temperature. CN102417769A discloses a method for producing an antibacterial coating using potassium silicate, sodium hexametaphosphate, zinc oxide, copper oxide, chloroethylene-vinylidene chloride emulsion, and a vinyl-acrylic emulsion. CN107353424A discloses a high-polymer, iodine-containing, surface-antibacterial plastic and a method for its production. JPH1112478A discloses the treatment of a plastic surface of an already molded plastic with an antibacterial resin.US20150273755A1 aims, among other things, at providing plastic surfaces with nanometer-sized structures that impart bactericidal properties to the surface. DE102012210807A1 describes the vapor deposition of an antibacterial surface. The method relates to the production of a bactericidal layer on a base body made of metal, glass, ceramic, textile, but preferably of titanium or a titanium-based alloy. A silver-containing, copper-containing, zinc-containing, or bismuth-containing silicon oxide layer is produced on the base body by means of plasma-assisted chemical vapor deposition under atmospheric pressure conditions, with the incorporated silver, copper, zinc, or bismuth being present in particulate or ionic form. No document refers to the production of an antibacterial surface on plastic as part of a pretreatment process during a plastics processing.The object of the invention is to provide a method and a device for the plasma pretreatment of plastic molded parts made of particulate plastic polymers, which enables an efficient activation of the surface of the plastic molded part and a bonding of the particulate plastic polymers with previously unbondable materials with little effort.

[0015] This object is achieved with the features specified in claim 1. The method according to the invention describes the processing of particulate plastic polymers (PKP) in a molding tool with one or more cavities and one or more injector nozzles for injecting the PKP into the cavity or cavities. Plasma generated by a plasma or corona discharge is applied to the PKP before it is transported into the tool. Plasma treatment is the generic term for both treatment methods.

[0016] In contrast to the prior art, plasma treatment is neither carried out directly in the tool, for example by introducing a plasma stream, nor by post-treatment of the semi-finished or final product produced by forming or primary shaping. Rather, the invention relates to the treatment of the PKP before entering or filling the tool cavity. The PKP are passed through a plasma container or pretreatment chamber. This is located between the feed unit attached to the processing machine and the injector nozzle installed in the tool or - in the case of multiple nozzles - the upstream distributor. As the plasma is passed through, the surface energy of the particles increases. This increased surface activity results in the individual PKPs having significantly better adhesion properties both to each other and to the particle surface itself.This allows inactive materials such as polypropylene or polyetheretherketone to be processed more energy-efficiently, or generally creates the conditions for their processing. A particular advantage is that the pretreatment takes place at a stage when the polypropylene can be easily treated, making the process significantly more effective overall. The new process therefore saves cycle time in the product manufacturing process and eliminates an additional processing step for surface treatment. The potential is also increased by the fact that this innovative, environmentally friendly process eliminates joining processes such as bonding and the use of adhesion-promoting films. End-of-life recycling is significantly simplified, and the integration of plasma treatment into the processing plant leads to more cost- and cycle-efficient production of complex components.Bonding the PKP with antibacterial materials before injection into the cavity is only possible through pretreatment. Plasma treatment can also be a corona treatment. For this purpose, corona nozzles are used instead of plasma nozzles. The corona process usually takes place under atmospheric pressure. One advantage of indirect corona treatment is the very low heat input into the surface. This makes this technology ideal for the pretreatment of heat-sensitive substrates such as PKP and, in particular, granules for particle foaming.

[0017] The PKP enter a plasma unit through a metering valve and then an inlet tube. In the inlet tube, they are irradiated with plasma by at least two opposing plasma nozzles, which are irradiated with plasma at an angle of 1 to 89° relative to the inlet tube, thereby activating the plasma. From there, they are pumped into an injector nozzle by an air venturi pump using negative pressure. From there, they are also pumped into the mold using the air venturi pump. This is the filling process. Subsequently, the previously activated particulate plastic polymers are stiffened in the mold using steam or a similar process, as is well known in the art, with the injector nozzle's plunger closed.

[0018] The PKP are then pumped through a particle container with a second dosing valve located upstream of the injector nozzle. Collecting them in the container allows them to be decoupled from the molding process in the cavity. While a workpiece is being created there, the PKP can be further activated and collected for the next molding process. In a further preferred process, materials, particularly antibacterial materials such as silver, copper and zinc, are added to the plasma stream, which then deposit on the PKP. This can, among other things, increase the surface binding capacity of the PKP and ultimately of the finished molded part. Coatings for special applications are also possible without the need for extensive post-treatment of the already formed tool. In this way, for example, antibacterial surfaces for medical-technical applications or products with special hygiene requirements can be produced.To date, chemical vapor deposition has been used primarily in the electronics industry. Outside the electronics industry, applications include the finishing of glass and plastics. The application of optical coatings to plastics is relevant (Materials World, Vol. 11, No. 5, pp. 13-15, May 2003). Boron-doped CVD diamond electrodes are used, among other things, in industrial water treatment for wastewater oxidation and the disinfection of process water. Organosilicon compounds can also be used as coating materials. This was successfully demonstrated in a test conducted by the inventors using a Tigres T-Jet Duosystem, STS 10.2, Direct Mode Coating. Organosilicon compounds are compounds that contain silicon-carbon bonds or in which the carbon is bonded to the silicon via oxygen, nitrogen, or sulfur atoms.They can be used, among other things, as hydrophobic agents (organosilanols such as trimethylsilanol, organochlorosilanes), as lubricants for plastics processing, as sealing materials, as sealing materials (silicon surfactants) and as adhesion promoters (organofunctional silanes).

[0019] In a further preferred process, the particulate plastic polymers are transported into the mold in cycles. The PKP are transported into the mold in cycles to allow for decoupling from the activation process. This has advantages in production, primarily ensuring a supply of PKP and ensuring smooth molding in the mold.

[0020] In a further preferred process, the particles are accelerated by the plasma radiation, so that no additional transport mechanism is required. This represents a further process optimization. Transport and treatment are carried out by one and the same mechanism. This represents a further process optimization. Transport and treatment are carried out by one and the same mechanism.

[0021] In a further preferred process, the plasma-treated PKPs are bonded to other materials such as foils, metal sheets, or nonwovens in the forming tool. This allows the surfaces to be given one or more new properties without any subsequent treatment. Plasma pretreatment enables the adhesion of the PKPs to these materials. Previously, plasma treatment required a separate process step. Furthermore, new material combinations with plastics, metals, and natural fiber products (wood, wood products) are possible.

[0022] In a further preferred process, the molded parts produced by pretreatment are further processed through subsequent processing steps, particularly bonding, painting, coating, and printing. In combination with the new pretreatment technology, this results in advantages because a process step can be omitted.

[0023] In a further preferred embodiment, additional plastics treatment processes such as injection molding, particle foaming, reactive foaming, and thermoforming are applied in the mold. The plasma-treated PKP can, among other things, be back-foamed or back-injected into the cavity(ies) without further surface pretreatment. This allows a solid composite with the desired properties to be created. The resulting materials have a pre-activated surface, eliminating the need for complex post-processing.

[0024] Last but not least, a device based on the aforementioned methods is disclosed. This device consists of at least the mold and an upstream plasma unit in which the particulate plastic polymers are subjected to plasma flow. The PKP is processed according to the methods described above. Optionally, a separate plasma generation unit, a particle container, a no-injection nozzle, and / or a plasma deposition unit are available.

[0025] The invention will be illustrated in the following exemplary embodiment. The plasma treatment of the individual particles is integrated into a particle foaming process. The particulate plastic polymers (PCP) (1) are supplied from the central storage through a metering valve (18) into an inlet pipe (19) of a plasma unit (3). The plasma nozzles (10) of the plasma unit (3) irradiate the PCP (1) with plasma at a 45° angle. The PCP (1) initially form a plasma-particle mixture. Subsequently, the activated PCP (1) are pumped into a particle container (4) by means of negative pressure. From there, they are pumped for injection through a second metering valve (20) into the injector nozzle (2) by means of a negative pressure applied there by a Venturi nozzle (12), and from there in cycles through a third metering valve into the cavity (5). The closure piston (15) closes as in Fig.2, so that the bonding of the PKP (1) with steam in the cavity (5) can begin. The produced product can then be further processed in the cavity (5) or removed from it for further processing. During processing, plasma flows around more PKP (1) through the plasma nozzles (10) and collects it in the particle container (4). The closure piston (15) of the injector nozzle (2) is opened again, allowing more PKP (1) to flow in for the next processing step. Sources: Innovent e. V. Technology development, Galvanotechnik 8 / 2015, Eugen G. Leuze Verlag. Gerullis, Sven et al., Thin antimicrobial silver, copper or zinc containing SiOx films on wood polymer composites (WPC) applied by atmospheric pressure plasma chemical vapor deposition (APCVD) and sol-gel technology, Eur. J. Wood Prod, online August 28, 2017. List of reference symbols 1 Particulate plastic polymers 2 injector nozzle 3 Plasma unit 4 particle containers 5 Cavity with molded part 6 Contour insert injector side 7 Contour insert core side 8 Machine plate injector side - fixed 9 Machine plate core side - movable 10 plasma nozzles 11 Addition of additives 12 Air supply injector nozzle for particle transport 13 Control for closing piston 14 filler neck 15 locking pistons 16 Plasma particle mixture 17 housing plasma nozzles 18 Dosing valve 19 Inlet pipe 20 Second dosing valve

Claims

[1] Particle foaming process for processing particulate plastic polymers (1) in a molding tool (5, 6, 7, 8, 9) having one or more cavities (5) and one or more injector nozzles (2) for injecting the particulate plastic polymers (1) into the cavity or cavities (5), characterized by, that the particulate plastic polymers (1) enter a plasma unit (3) through a metering valve (18) and subsequently an inlet pipe (19) before injection into the cavity, in which they are surrounded by plasma through at least two opposing plasma nozzles (10), which are arranged at an angle of 1 to 90° relative to the inlet pipe (19), are pumped from there into a particle container (4) and collected there, from there into an injector nozzle (2) with a second metering valve (20) located in front of the injector nozzle (2) by means of a vacuum pump (12) and from there also with the help of the air venturi pump (12) into the tool (5, 6, 7, 8, 9). [2] Method according to claim 1, characterized by , that materials, in particular antibacterial materials such as silver, copper and zinc, are added to the plasma stream, which are deposited on the particulate plastic polymers (1). [3] Method according to any one of the preceding claims, characterized by , that the particulate plastic polymers (1) are transported in cycles into the tool (5, 6, 7, 8, 9). [4] Method according to any one of the preceding claims, characterized by , that the plasma-treated, particulate plastic particles (1) are combined with other materials such as foils, metal sheets or nonwovens in the shaping tool (5, 6, 7, 8, 9). [5] Method according to any one of the preceding claims, characterized by , that the molded parts produced by the pretreatment are further processed by subsequent processing steps, in particular gluing, painting, coating and printing.

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