【発明の詳細な説明】
本発明は熱可塑性樹脂粉末の製造方法に関す
る。熱可塑性樹脂粉末はホツトメルト接着、粉体
塗装等に多く使用される。例えば粉体塗装方法は
省資源、公害防止、塗膜性質等から優れた塗装方
法であるとされている。粉体塗装の応用される分
野を大きく二つに分けると美観塗装と防蝕、電気
絶縁、耐摩擦耗、耐候等の機能を目的とする塗装
に分れる。機能を目的とする塗装には熱可塑性樹
脂粉体、特にポリアミド系樹脂粉体の流動浸漬塗
装が広く用いられる。
流動浸漬塗装に用いられる粉末の製造方法とし
ては熱可塑性樹脂を該樹脂を溶解又は膨潤させる
溶媒中で溶解し、次に沈澱させて粉末状にするケ
ミカル粉砕と称せられる方法がある。該方法で得
られる粉末は粒径がそろつており、かつ粉末形状
がほぼ球形のため、非常に流動性が良く、流動浸
漬塗装には最適の粉砕方法である。しかしながら
ケミカル粉砕方法では顔料又は充填物を均一に分
散されて含有する熱可塑性粉末を得にくい。又ケ
ミカル粉砕方法は溶剤の揮発、回収等設備及び粉
砕に多額の費用を要する。
他方、いまひとつの方法である機械式冷凍粉砕
する方法は設備費も低額ですみ、又容易に清掃で
きる為、安価に多品種の粉末を製造することがで
きる。着色はあらかじめ押出機等を使用して混練
する方法により染顔料の分散を充分行なうことが
できる。すなわち、熱可塑性樹脂を押出機を用い
染顔料安定剤を混練し、ストランドカツト方式、
すなわちダイス部分から押出された樹脂をストラ
ンドにして自然冷却し、冷水中を通してから切断
する方式によつてペレツト化し、このペレツト化
された物（以下ペレツト）を液体窒素等の冷媒で
冷却してピンミル又はハンマーミル等の機械式粉
砕機により粉砕する方法である。
しかしながら、通常一般に用いられる上記スト
ランドカツト方式のペレツトを機械粉砕した場合
得られる粉末の形状は球形になることはなく、直
方体又は板状等細長い形状の物が多く、形状が一
定していない。
このため流動浸漬を行なつた場合の流動性はケ
ミカル粉砕法に比較してかなり低下する。
流動浸漬塗装において流動性が低下すると微粉
末の飛散による環境悪化、又は塗装膜厚の不均一
等の製品品質低下をまねく。
本発明者は上記問題点、特に粉末の形状を改善
する方法を鋭意検討を行い、本発明に到達した。
すなわち、熱可塑性樹脂を押出機のダイス孔より
該樹脂の溶融点以上の樹脂温度で50〜100℃の温
水中に直接押出し、温水中にて切断してほぼ球状
の樹脂粒子を得、この樹脂粒子を冷凍粉砕するこ
とを特徴とする流動性の改善された粉末の製造方
法に関する。
本発明における熱可塑性樹脂とはナイロン６、
ナイロン6.6、ナイロン6.12、ナイロン11、ナイ
ロン12およびこれらの共重合体、ポリエチレン、
エチレン酢酸ビニル共重合物、ポリ塩化ビニル等
をさし、安定剤、染顔料等の添加をこばむもので
はない。塗膜性能の点ではナイロン12、ナイロン
11が特に優れており、通常一般に用いられる機会
が多い。
本発明における押出方法では温水中で樹脂をダ
イス部から直接温水中に押し出し、樹脂が結晶化
する前に切断するアンダーウオーターカツト方式
を特徴とする。この方法によれば、押出歪が無く
ほとんど無配向のほぼ球状のペレツトが得られ
る。
本発明では押出機のダイス部の樹脂温度は該樹
脂の融点以上であるが、好ましくは該樹脂の融点
＋30℃〜融点＋100℃の範囲が良好である。樹脂
温度が低すぎる場合は押出歪が残りやすく、すな
わち、ほぼ球状のペレツトが得にくい。逆に樹脂
温度が高すぎると切断が難かしくなり、ヒゲ状の
物が多くなり、粉砕後の流動性に悪影響を及ぼ
す。
冷却するための温水温度は50〜100℃であり、
50℃未満ではダイス部から出た樹脂が急に冷却さ
れるため歪（ペレツト内部に押出し方向の応力、
特の圧縮応力）が残りやすく、そのため粉砕時の
ペレツトの破壊に方向性が表われ粉末形状が球形
から遠くはなれることになり、流動性が悪くなる
切断時期も樹脂が押し出されてから早い時期に行
なうのが良く、切断後にペレツトの押出し歪、内
部応力ができるだけ緩和されるように押し出され
て即時行なうのが好ましい。
本発明に使用できる押出し機としては、通常一
般に用いられる単軸又は多軸押出し機が用いられ
る。
本発明で用いられる冷凍粉砕機としては、ハン
マーミル、ピンミル等の一般に用いられるものが
使用できるが、好ましくは発熱の少ないピンミル
方式が良い。
以下実施例・比較例によつて本発明を説明す
る。
実施例 １
ナイロン12をスクリユー式押出機により温水中
に押し出し、即時切断することにより粒径約３mm
の球状ペレツトを製造した。
押し出し条件はシリンダー後部240℃、中部230
℃、前部230℃、ダイス部樹脂温度240℃、温水温
度90℃にし、温水中にて４枚刃の回転式カツター
により回転速度1250回転／分で切断した。
上記ペレツト液体窒素を冷媒としてピンミル方
式の冷凍粉砕機により粉砕し、250μのふるいで
分級し、ふるいを通過した粉末により流動浸漬塗
装を行なつた。粉砕温度は−90〜80℃であり、ピ
ンミルの回転数は8000回転／分である。結果を表
−１に示す。
比較例 １
実施例１と同様のナイロン12ペレツトをスクリ
ユー式押出機により押し出し、ストランドを80℃
の温水中で充分冷却し、ペレタイザーでカツテイ
ングすることにより、円筒形のペレツトを製造し
た。押し出し条件は実施例１と同様であり、冷凍
粉砕条件、分級も実施例１と同一条件で行ない、
得られた粉末を流動浸漬塗装を行なつた。結果を
表−１に示す。
【表】DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing thermoplastic resin powder. Thermoplastic resin powder is often used for hot melt adhesion, powder coating, etc. For example, powder coating is said to be an excellent coating method due to its resource saving, pollution prevention, coating film properties, etc. The fields in which powder coating is applied can be roughly divided into two categories: aesthetic coatings and coatings for functional purposes such as corrosion prevention, electrical insulation, abrasion resistance, and weather resistance. Fluid-dip coating of thermoplastic resin powder, especially polyamide resin powder, is widely used for functional coatings. As a method for producing powder used in fluidized dip coating, there is a method called chemical pulverization, in which a thermoplastic resin is dissolved in a solvent that dissolves or swells the resin, and then precipitated to form a powder. The powder obtained by this method has a uniform particle size and a nearly spherical powder shape, so it has very good fluidity and is the most suitable pulverization method for fluidized dip coating. However, it is difficult to obtain a thermoplastic powder containing uniformly dispersed pigments or fillers using the chemical grinding method. In addition, the chemical pulverization method requires a large amount of equipment for solvent volatilization, recovery, etc., and pulverization. On the other hand, another method, mechanical cryo-pulverization, requires low equipment costs and is easy to clean, making it possible to produce a wide variety of powders at low cost. For coloring, the dye and pigment can be sufficiently dispersed by kneading in advance using an extruder or the like. That is, thermoplastic resin is kneaded with dye and pigment stabilizers using an extruder, and then the strand cut method is used.
In other words, the resin extruded from the die is made into strands, cooled naturally, passed through cold water and cut into pellets, and the pelletized material (hereinafter referred to as pellets) is cooled with a refrigerant such as liquid nitrogen and made into a pin mill. Alternatively, it is a method of pulverizing with a mechanical pulverizer such as a hammer mill. However, when the commonly used pellets of the strand cut method are mechanically pulverized, the shape of the powder obtained is not spherical, but is often elongated, such as a rectangular parallelepiped or a plate, and the shape is not uniform. For this reason, the fluidity when fluidized immersion is performed is considerably lower than that when using chemical pulverization. A decrease in fluidity in fluidized dip coating can lead to environmental deterioration due to the scattering of fine powder, or to product quality deterioration such as uneven coating film thickness. The inventors of the present invention have conducted extensive studies on methods for improving the above-mentioned problems, particularly the shape of the powder, and have arrived at the present invention.
That is, a thermoplastic resin is directly extruded through a die hole of an extruder into hot water of 50 to 100°C at a resin temperature higher than the melting point of the resin, and cut in the hot water to obtain approximately spherical resin particles. The present invention relates to a method for producing powder with improved fluidity, which comprises freezing and pulverizing particles. The thermoplastic resin in the present invention is nylon 6,
Nylon 6.6, nylon 6.12, nylon 11, nylon 12 and their copolymers, polyethylene,
It refers to ethylene vinyl acetate copolymer, polyvinyl chloride, etc., and does not require the addition of stabilizers, dyes, pigments, etc. Nylon 12, nylon in terms of coating performance
11 is particularly good and is often used in general. The extrusion method of the present invention is characterized by an underwater cut method in which the resin is directly extruded from a die portion into hot water and cut before the resin crystallizes. According to this method, almost spherical pellets with no extrusion strain and almost no orientation can be obtained. In the present invention, the temperature of the resin in the die section of the extruder is higher than the melting point of the resin, preferably within the range of +30°C to +100°C of the melting point of the resin. If the resin temperature is too low, extrusion distortion tends to remain, that is, it is difficult to obtain approximately spherical pellets. On the other hand, if the resin temperature is too high, it will be difficult to cut and the number of whiskers will increase, which will adversely affect the fluidity after pulverization. The hot water temperature for cooling is 50-100℃,
At temperatures below 50°C, the resin released from the die is cooled rapidly, causing distortion (stress in the extrusion direction inside the pellet,
(particularly compressive stress) tends to remain, and as a result, the pellets break directionally during pulverization, causing the powder shape to deviate from a spherical shape, and the cutting time at which flowability deteriorates is also early after the resin has been extruded. It is preferable to extrude the pellet immediately after cutting so that the extrusion strain and internal stress of the pellet are alleviated as much as possible. As an extruder that can be used in the present invention, a commonly used single-screw or multi-screw extruder can be used. As the cryo-pulverizer used in the present invention, commonly used ones such as a hammer mill and a pin mill can be used, but a pin mill type which generates less heat is preferably used. The present invention will be explained below with reference to Examples and Comparative Examples. Example 1 Nylon 12 was extruded into warm water using a screw extruder and immediately cut to a particle size of approximately 3 mm.
spherical pellets were produced. Extrusion conditions are 240℃ at the rear of the cylinder and 230℃ at the middle.
℃, the front part temperature was 230℃, the die part resin temperature was 240℃, and the hot water temperature was 90℃, and cutting was performed using a four-blade rotary cutter at a rotational speed of 1250 revolutions/minute in hot water. The above pellets were pulverized by a pin mill type freezing pulverizer using liquid nitrogen as a refrigerant, classified with a 250μ sieve, and fluidized dip coating was performed using the powder that passed through the sieve. The grinding temperature is -90 to 80°C, and the rotation speed of the pin mill is 8000 revolutions/min. The results are shown in Table-1. Comparative Example 1 Nylon 12 pellets similar to those in Example 1 were extruded using a screw extruder, and the strands were heated to 80°C.
Cylindrical pellets were produced by cooling sufficiently in hot water and cutting with a pelletizer. The extrusion conditions were the same as in Example 1, and the freeze-grinding conditions and classification were also carried out under the same conditions as in Example 1.
The obtained powder was subjected to fluidized dip coating. The results are shown in Table-1. 【table】