I want to control infinitely generated Prefab material with time.

In Unity3D, I am creating a script that generates infinite Prefabs.
I want to loop the Prefab material from Material1 to Material2 to Material3 to Material1 at regular intervals.
Infinite generation of Prefab could be performed, but it was clogged at the time control stage of the material.

Error message

Error message

Applicable source code

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
public class CreateMultiple: MonoBehaviour {
    public Material [] _material;// Material to assign.
    float time = 5;
    // Use this for initialization
    void Start () {
    }
    // Update is called once per frame
    void Update () {
        for (int i = 0;i<20;i ++) {
                time-= Time.deltaTime;
                if (time<= 0)
        {
                     // Create an instance by passing Prefab as an argument of Instantiate
                     GameObject obj = (GameObject) Resources.Load ("CarPrefab");
                     // place in a random location
                     Instantiate (obj, new Vector3 (Random.Range (250, 4), 5, Random.Range (100, 4)), Quaternion.identity);
                     time = Random.Range (3, 6);
             }
         }
    }
}

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Supplemental information (FW/tool version etc.)

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